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Safe to run 5W30 in the summer?

1453 Views 28 Replies 8 Participants Last post by  itsthemetho
First year owners have mentioned to me a few times they run 0W-20 in the off season and 5W-30 for the track season (even Auto X).

I live in the north so we see cold winters. Below freezing. But in the summer I was planning on now changing over to 5W30. I get the silly manual says to use 5W20/30 and then switch it back to 0W10.

Just trying to see if it鈥檚 worth trying it out to ensure good oil pressure while doing HPDE.
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I'm waiting on @TRU-BOOST entry.

i run 5w30 almost all year around. But i do live in warmer climate. North Carolina to be exact.

5w30 will def help with how much heat this engine generates on track. But oil weight is not a magic bullet to combat heat, without a pressure gauge you don't know what's exactly going on inside the engine. So if you do track the car stock, when you see the temperature gets up to 250 - 270 range i would back off and let the car cool down.

But through @TRU-BOOST persistence recommendation, lots of people run 5w20 for DD on here. Sought to be a lot more stable than 0w20 (i dont even know if im describing it right, i aint no expert). So take that however you want 馃憤
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Depending on how far north 5w30 is more than likely overkill for anything other than track use, even in summer. I am in Chicago area and run 5w20 all year on and off track. So far I havent made it to the "I blew my motor" section. YRMV
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I'm on 0W-40 and my BRZ is subjected to both cold (negative teens C) and hot (above 33C) throughout the year. If and only if you are going to stay on top of oil changes based on season would I use 0W in the winter and 5W in the summer. Personally, I would just use 0W all year long. Some will say the difference between 0W and 5W isn't applicable until -30C because the viscosity is the same at that temperature, but that doesn't account for the viscosity curve as the oil warms up. You do not need to live in an area that sees -30C to get the benefits of a 0W, which is increased pumpability while the engine is warming. Pick whatever hot viscosity grade you feel comfortable with. Different people have different opinions on hot viscosity grades. I use a 40 grade for the higher HTHS value that I have seen in my area of study/profession to be beneficial to engine wear. You can also look up "HTHS wear graph" and form an opinion of your own. The owner's manual also calls for ACEA A3 oils, which is at least 3.5 HTHS and it would pretty much always fall under a high 30 grade or low 40 grade.
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I'm on 0W-40 and my BRZ is subjected to both cold (negative teens C) and hot (above 33C) throughout the year. If and only if you are going to stay on top of oil changes based on season would I use 0W in the winter and 5W in the summer. Personally, I would just use 0W all year long. Some will say the difference between 0W and 5W isn't applicable until -30C because the viscosity is the same at that temperature, but that doesn't account for the viscosity curve as the oil warms up. You do not need to live in an area that sees -30C to get the benefits of a 0W, which is increased pumpability while the engine is warming. Pick whatever hot viscosity grade you feel comfortable with. Different people have different opinions on hot viscosity grades. I use a 40 grade for the higher HTHS value that I have seen in my area of study/profession to be beneficial to engine wear. You can also look up "HTHS wear graph" and form an opinion of your own. The owner's manual also calls for ACEA A3 oils, which is at least 3.5 HTHS and it would pretty much always fall under a high 30 grade or low 40 grade.
Unfortunately it doesnt really work the way you think it does. Your 0w40 is actually thicker at almost all temps than a 10w30. Just because it starts with a 0 does not mean they are all the same at low temps. You are doing your car a disservice in cold temps. That stuff is flowing way slower than you think it is on a car that does not have gaping oil passages like european cars do.
Unfortunately it doesnt really work the way you think it does. Your 0w40 is actually thicker at almost all temps than a 10w30.
You can not confidently say that without viscosity curves and comparing two oils side by side. Even 2 different 5W-20 oils can have completely different viscosity curves if you plot their viscosity between all temperatures at cold cranking -30C tests all the way up to cSt @ 40C and cSt @ 100C.

You are doing your car a disservice in cold temps. That stuff is flowing way slower than you think it is on a car that does not have gaping oil passages like european cars do.
At cold start and only at cold starts. Once the engine is actually running and temperature is beginning to climb, flow is the same regardless of viscosity in a positive displacement oil pump engine. Oil will get to where it needs to go because of the pump.
You can not confidently say that without viscosity curves and comparing two oils side by side. Even 2 different 5W-20 oils can have completely different viscosity curves if you plot their viscosity between all temperatures at cold cranking -30C tests all the way up to cSt @ 40C and cSt @ 100C.


At cold start and only at cold starts. Once the engine is actually running and temperature is beginning to climb, flow is the same regardless of viscosity in a positive displacement oil pump engine. Oil will get to where it needs to go because of the pump.
I can confidently say that. In almost all cases that holds true. A few outliers here and there and it will vary if you compare brand A 10w30 to brand B 0w40. But within the same brand and product line the 2 perform nearly identical until much higher temps.

Yes a positive displacement pump will move pretty much any viscocity oil. It will take much longer to get moving at initial startup which is already where most engine wear occurs. It will also force the oil filter bypass open on a regular basis releasing all that shit that it caught right into your engine.
I love some extra visc, but I think you are over doing it.
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I can confidently say that. In almost all cases that holds true. A few outliers here and there and it will vary if you compare brand A 10w30 to brand B 0w40. But within the same brand and product line the 2 perform nearly identical until much higher temps.

Yes a positive displacement pump will move pretty much any viscocity oil. It will take much longer to get moving at initial startup which is already where most engine wear occurs. It will also force the oil filter bypass open on a regular basis releasing all that shit that it caught right into your engine.
I love some extra visc, but I think you are over doing it.
It's okay. We can agree to disagree. Even within the same brand, the viscosity, content of VII/VM, etc. all differ drastically. Using oil grades as way to pick oil is very antiquated and has been under scrutiny for many years because it tells you very little about an oil. That's why so many of the European manufacturers started to use approvals. Not all xW-y oil is the same, but approvals tell you, with actual data and testing, that the oil you chose passed certain tests to become approved. Looking at things like base oils, approvals, VII content, viscosity curves, etc. tells you much more about an oil than grades ever will.

Oil bypass works on a delta between the two sides, not as an absolute pressure. Even at WOT, filter bypass is a non-issue. The OEM filters also have high bypass pressure anyway, north of 25 PSI. You won't see a 25 PSI delta for more than fractions of a second, faster than the bypass can function and swap between filter and full bypass. The bypass is primarily there to ensure oil pressure if there is catastrophic failure between the input and output side of the filter.
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I can tell you there is a reason we dont deal with the european ratings and certifications. THEY ARE ALL NUTS. They design poor quality, inferior powertrains and then want to put it in the hands of the lubrication companies to make them reliable. I am still convinced that while cold your filter bypass will be open longer than you think. You will be seeing elevated pressure on both sides of the filter as that thick oil tries to make its way through all those passages that were designed for much thinner oil.
I wish subaru was an OEM I dealt with to know for sure, but I am just basing that off of experience with other platforms.
Either way, your experience will be good data. Wish you well.
I can tell you there is a reason we dont deal with the european ratings and certifications. THEY ARE ALL NUTS. They design poor quality, inferior powertrains and then want to put it in the hands of the lubrication companies to make them reliable. I am still convinced that while cold your filter bypass will be open longer than you think. You will be seeing elevated pressure on both sides of the filter as that thick oil tries to make its way through all those passages that were designed for much thinner oil.
I wish subaru was an OEM I dealt with to know for sure, but I am just basing that off of experience with other platforms.
Either way, your experience will be good data. Wish you well.
Been using 0W-40 on my two Ascents FA24F for the last 3 years as well as this BRZ for the last year and, so far, it's been serving me well. More time will tell.

To my understanding, the Euro manufacturers created the approvals because they are not formulators. They don't manufacture or formulate oil, so relying on lubrication companies is a given. Tons of American and Japanese vehicle manufacturers also have builder approvals, Chrysler and Honda being the most prominent. Simply having oil approvals does not mean their engines are relying on the oil to be reliable. A lot of the Euro specs come from having access to the autobahn where there are longer stretches of road being driven at higher speeds as well as both gasoline and diesel use. There are timing chain wear tests, LSPI tests, stay-in-grade tests, etc. etc. which are applicable to all vehicles regardless of country of origin. Manufacturer approvals are the only objective way of measuring oil performance because the oils are actually being used and tested in a real engine and not just numbers on a paper.

The oil pump may be in relief longer on cold starts, but elevated pressure on both sides of the oil filter would actually cause less of a chance of bypass. It's the difference between the two pressures that cause the bypass valve to open, not the total pressure.

If Subaru knew there was a problem with thick oil, they would probably say so. Instead, they state the exact opposite. They recommend using API SN Plus Resource Conserving oil OR ILSAC GF-5 oil OR ACEA A3 or A5. ACEA A3 or A5 are going to be HTHS >=3.5 or HTHS 2.9-3.5 respectively, both specifications found in high 30 grade or low 40 grade oils and not found in 20 grade oils.


Anyways, tribology is a touchy subject and there's a lot of mystery in it. My opinion is formulated from my experience in NDT and working on ICEs and oils every day. Your opinion may differ due to your experiences. I'll continue to use what works for me and makes sense to me.
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@TessaSeveride, are you aware that @TRU-BOOST works in the industry and tests oils for a living? lol

Not saying you're wrong, but you are arguing with a professional in that field.
@TessaSeveride, are you aware that @TRU-BOOST works in the industry and tests oils for a living? lol

Not saying you're wrong, but you are arguing with a professional in that field.
My opinion is formulated from my experience in NDT and working on ICEs and oils every day.
I am also a professional in the field.
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Missed your comment below that graphic, but understood.

Do you test oils as he does in a lab environment? I'd like to hear a little more about your background.
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@TessaSeveride, are you aware that @TRU-BOOST works in the industry and tests oils for a living? lol

Not saying you're wrong, but you are arguing with a professional in that field.
Its all good. This is discussion and no argument. Certainly not from me and I dont get that vibe from @TessaSeveride.
I learn stuff from others on here regularly and have no shame to admit that. What I have said before and maybe should point out is that I manufacture viscocity modifiers, additives, detergents among many other specialty chemicals that arent automotive related at all. We do supply many of the worlds biggest names with the products used in their formulas. I know the chemistry and behavior of the oils themselves very well. I will not claim to be an expert on engines themselves, filter types\media, internal engine designs, etc. The OEMs have their engineers for that. And unfortunately Subaru is not a customer. I base everything I say here on experiences with other brands. It is very possible for me to miss the mark with this platform due to my lack of experience with it.
Moral of the story is I try my best to be accurate, but never hesitate to correct me if you know better. 馃憤
Its all about us learning what these cars like to avoid being the OP of the next "oops it blew up" thread.
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Missed your comment below that graphic, but understood.

Do you test oils as he does in a lab environment? I'd like to hear a little more about your background.
My company does mostly government contract work in a lab environment. We do both invasive and non-invasive testing to determine the failure point of engines among other things like metrology and engineering. I personally work in NDT where I image, disassemble, measure, and test engines to failure. My colleagues in my department have similar jobs where they do what I do except with satellites, aircraft, consumer electronics, etc.
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Its all good. This is discussion and no argument. Certainly not from me and I dont get that vibe from @TessaSeveride.
I learn stuff from others on here regularly and have no shame to admit that. What I have said before and maybe should point out is that I manufacture viscocity modifiers, additives, detergents among many other specialty chemicals that arent automotive related at all. We do supply many of the worlds biggest names with the products used in their formulas. I know the chemistry and behavior of the oils themselves very well. I will not claim to be an expert on engines themselves, filter types\media, internal engine designs, etc. The OEMs have their engineers for that. And unfortunately Subaru is not a customer. I base everything I say here on experiences with other brands. It is very possible for me to miss the mark with this platform due to my lack of experience with it.
Moral of the story is I try my best to be accurate, but never hesitate to correct me if you know better. 馃憤
Its all about us learning what these cars like to avoid being the OP of the next "oops it blew up" thread.
It's best to stay open minded, but I didn't mean to imply a heated exchange, although anymore most tend towards the definition of argument having that component vs a calm discussion with opposing views. But I guess most "arguments" these days do have a negative implication.

My company does mostly government contract work in a lab environment. We do both invasive and non-invasive testing to determine the failure point of engines among other things like metrology and engineering. I personally work in NDT where I image, disassemble, measure, and test engines to failure. My colleagues in my department have similar jobs where they do what I do except with satellites, aircraft, consumer electronics, etc.
Awesome, I wasn't questioning credibility just was more curious about your field vs his.
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Its all good. This is discussion and no argument. Certainly not from me and I dont get that vibe from @TessaSeveride.
I learn stuff from others on here regularly and have no shame to admit that. What I have said before and maybe should point out is that I manufacture viscocity modifiers, additives, detergents among many other specialty chemicals that arent automotive related at all. We do supply many of the worlds biggest names with the products used in their formulas. I know the chemistry and behavior of the oils themselves very well. I will not claim to be an expert on engines themselves, filter types\media, internal engine designs, etc. The OEMs have their engineers for that. And unfortunately Subaru is not a customer. I base everything I say here on experiences with other brands. It is very possible for me to miss the mark with this platform due to my lack of experience with it.
Moral of the story is I try my best to be accurate, but never hesitate to correct me if you know better. 馃憤
Its all about us learning what these cars like to avoid being the OP of the next "oops it blew up" thread.
No argument at all here. Tribology is very tricky and not something that is fully understood. There are even some studies that show shorter OCI will actually cause higher wear numbers in a UOA. It all comes down to who does the study and how the numbers are interpreted.

Awesome, I wasn't questioning credibility just was more curious about your field vs his.
Oh, no worries. I didn't take it like that at all.
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There are even some studies that show shorter OCI will actually cause higher wear numbers in a UOA.
I was actually discussing this with my brother not long ago and we both couldn't really surmise why that would be the case, but I have heard of that several times in the past.
I was actually discussing this with my brother not long ago and we both couldn't really surmise why that would be the case, but I have heard of that several times in the past.
The theory is that fresh oil puts a tribofilm layer on all the metal that the old oil already laid down, so the oil film that was layered on top of all the metal parts in the engine previously gets replaced. During that replacement process, there are times when there might be a smaller or non-existent layer of tribofilm protecting the wear points, which means less protection and higher wear. The study noticed the "increased wear" by increased iron levels in UOA at shorter OCI. The study also included a few short OCI and a few longer OCI and showed that wear indicators tapered off after going to a longer OCI. To them, this proved that a short OCI caused higher wear.

My theory is that there isn't any accelerated wear going on. It's just that some of the iron was trapped on the tribofilm layer and the new oil simply replaces that layer so the iron falls into suspension. I wish the study used different oils, maybe some with more AN or esters for higher solvency and less of a tendency to trap wear metals in the tribofilm layer. That way, if my theory is correct, there should at least be less wear metals to fall back into suspension when fresh oil is added.
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Depending on how far north 5w30 is more than likely overkill for anything other than track use, even in summer. I am in Chicago area and run 5w20 all year on and off track. So far I havent made it to the "I blew my motor" section. YRMV
Good old Massachusetts. Been using Pennzoil Ultra Platinum 5W-30. Mainly autocross car.
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