How much extra viscosity does 5w40 buy you over 0w20 in the 250-290 degree range?

[Lumberg]

So I track my car and I notice that (very generally, due to different atmospheric, tire, and overall conditions never being the exact same two sessions in a row, much less two different days separated by weeks), when I run a thicker oil, my oil temps are higher. Which begs the question, is running a 40w oil "worth it" over a 20 or 30, based on the fact that it actually raises oil temps, on average, about 20ish degrees?

The weights are defined by viscosity at 212F, but how does the relationship to viscosity hold up at higher temps? Is shell Rotella T6 5w40 producing more protection at 290 degrees than Mobil1 0w20 at 250? I'm trying to pick two extremes here, but is there a crossover point for where one is better than the other? Thanks!

 

[JavierH19]

Hths wise at 150c or 302f a 20 grade like that will be 2.7-2.8. But that's the overall oil temp you're seeing, not the temp at the most stressed point which will be higher. And at higher loads it matters more. I'd use a 40 grade when tracking since both temps and load is higher. Higher temps same load would mean overall less protection. Both combined is worse. And a euro 40 grade is going to have more zddp as well which helps for when the film thickness isn't enough. But even a 30 grade would be fine as a better option for both daily and track use. It wouldn't be much thinner than a 40. Only about 2 cst at kv100 thinner. If it were mine i'd just pick a euro 30 grade and use it for everything.

 

[Jetronic]

So I track my car and I notice that (very generally, due to different atmospheric, tire, and overall conditions never being the exact same two sessions in a row, much less two different days separated by weeks), when I run a thicker oil, my oil temps are higher. Which begs the question, is running a 40w oil "worth it" over a 20 or 30, based on the fact that it actually raises oil temps, on average, about 20ish degrees?

The weights are defined by viscosity at 212F, but how does the relationship to viscosity hold up at higher temps? Is shell Rotella T6 5w40 producing more protection at 290 degrees than Mobil1 0w20 at 250? I'm trying to pick two extremes here, but is there a crossover point for where one is better than the other? Thanks!

rule of thumb, so it's not accurate for ALL cases, you can go about 20)F higher for every grade up (so +40°F from xw-20 to xw-40) and end up with the same viscosity. so from a viscosity standpoint 0w-0 at 250 is the same as 5w-40 at 290 but there's other stuff going on (chemically) which gives the edge to the cooler oil.

 

[Greasymechtech]

Depends on engine and what it needs to survive

Grades are defined, not weights

And, don't assume that oil temp automatically raises with viscosity.... My running commute oil temps were 200F with 0w20, 5w30 and 5w40... I didn't measure a magical increase in oil temps. If I were to lose control of oil temps, I would definitely want as much visc as possible.

Years ago, Redline graphed all their oils. This allowed you to pick a new more suitable matching grade, when compared to temp, when engine mods increased those oil temps. I haven't seen that much 'detail' in marketing from Redline, or any other oil brand in 25 years, concerning visc curves of each specific grade.

Knowledge Centre | Penrite Oil

SAE stands for Society of Automotive Engineers. The SAE developed a numerical classification system to define the viscosity or thickness of oil. The original viscosity grades were all mono-grades, e.g. a typical engine oil was a SAE 30. This was an operating temperature viscosity. Basically, the...

penriteoil.com.au

 

[twX]

A 5W40 will have the same HTHS viscosity as a 0W20 when the 5W40 is hotter by roughly 20 degrees C. How much hotter is the thicker oil getting in your case?

 

[Lumberg]

rule of thumb, so it's not accurate for ALL cases, you can go about 20)F higher for every grade up (so +40°F from xw-20 to xw-40) and end up with the same viscosity. so from a viscosity standpoint 0w-0 at 250 is the same as 5w-40 at 290 but there's other stuff going on (chemically) which gives the edge to the cooler oil.

Thank you, this is a great rule of thumb! Understood that there's a lot more to it than that but it gives me a good starting point

 

[OVERKILL]

A 5W40 will have the same HTHS viscosity as a 0W20 when the 5W40 is hotter by roughly 20 degrees C. How much hotter is the thicker oil getting in your case?

I don't think it's that simple.

If you use a visc calc, if you plug in 150C, you do not get the same value as is reported for HTHS because KV doesn't present as a high shear scenario.
For example:
M1 5W-40:
KV40: 81cSt
KV100: 13.2cSt
HTHS: probably around 3.8cP
KV150: 5.47cSt
KV170: 4.20cSt

M1 5W-20:
KV40: 46.28
KV100: 8.42
HTHS: 2.6cP
KV150: 3.71cSt

Just using KV, not HTHS, our 5W-40 would have to be 30C hotter (180C) to be the same viscosity.

But, this is of course further complicated by VII content. We could guestimate a ratio. Take KV150 and divide it by HTHS (150).
- If I divide 5.47 by 3.8, I get 1.439
- If I divide 3.71 by 2.6, I get 1.427

Relatively close. We'll say 1.43

Using our 5W-40:
For KV170, 4.20cSt would yield an HTHS 170 of 2.93
For KV180, 3.73cSt would yield an HTHS 180 of 2.61

So, consistent with the 30C that we determined contrasting KV's.

Now, is this accurate more broadly? No idea, but one could test on a few examples and see if it tracks. 30C different is pretty big though. You could also run it the other way to determine the temp that the 5W-20 provided the same HTHS as the 5W-40 at 150C, which ends up being ~123C.

 

[Lumberg]

I don't think it's that simple.

If you use a visc calc, if you plug in 150C, you do not get the same value as is reported for HTHS because KV doesn't present as a high shear scenario.
For example:
M1 5W-40:
KV40: 81cSt
KV100: 13.2cSt
HTHS: probably around 3.8cP
KV150: 5.47cSt
KV170: 4.20cSt

M1 5W-20:
KV40: 46.28
KV100: 8.42
HTHS: 2.6cP
KV150: 3.71cSt

Just using KV, not HTHS, our 5W-40 would have to be 30C hotter (180C) to be the same viscosity.

But, this is of course further complicated by VII content. We could guestimate a ratio. Take KV150 and divide it by HTHS (150).
- If I divide 5.47 by 3.8, I get 1.439
- If I divide 3.71 by 2.6, I get 1.427

Relatively close. We'll say 1.43

Using our 5W-40:
For KV170, 4.20cSt would yield an HTHS 170 of 2.93
For KV180, 3.73cSt would yield an HTHS 180 of 2.61

So, consistent with the 30C that we determined contrasting KV's.

Now, is this accurate more broadly? No idea, but one could test on a few examples and see if it tracks. 30C different is pretty big though. You could also run it the other way to determine the temp that the 5W-20 provided the same HTHS as the 5W-40 at 150C, which ends up being ~123C.

Thank you for analyzing this more deeply.

I decided to go with Shell Rotella T6 5w40 and I will report back what, if any, temp rise I see at the track. Yesterday I was seeing mid-250s on Amsoil 0w20 (with some Rotella make-up oil) which I was comfortable with at the time, though not so much after reading this thread. So if the temps stay below 290 with the 5w40 I will consider that a win.

 

[Cujet]

It is not uncommon to see higher oil temps with a higher viscosity. And, each engine's response to temp rise with viscosity is different. Our turbocharged road race cars had adequate oil coolers and we would strive to keep oil temps well below 240F with a fresh quality synthetic before each race day.

Other teams would use 0W-20 synthetics and keep oil temps around 200, and I've heard of teams using 0W-2, 0W-8 and so on. Which requires a good bit more oil temp management. And of course some drag race engines that never get the oil warm use some very low viscosity oils.

The battle is as old as racing itself. I am solidly in the risk avoidance, adequate viscosity camp. The use of a lower viscosity may not in fact result in more HP or help you win the race, unless you need 1/16th of a MPH more on a 500 mile race.

 

[Lumberg]

It is not uncommon to see higher oil temps with a higher viscosity. And, each engine's response to temp rise with viscosity is different. Our turbocharged road race cars had adequate oil coolers and we would strive to keep oil temps well below 240F with a fresh quality synthetic before each race day.

Other teams would use 0W-20 synthetics and keep oil temps around 200, and I've heard of teams using 0W-2, 0W-8 and so on. Which requires a good bit more oil temp management. And of course some drag race engines that never get the oil warm use some very low viscosity oils.

The battle is as old as racing itself. I am solidly in the risk avoidance, adequate viscosity camp. The use of a lower viscosity may not in fact result in more HP or help you win the race, unless you need 1/16th of a MPH more on a 500 mile race.

Thanks for the insight and for validating that thicker oils can raise temps. I agree that going thin for power is sacrificing a lot of risk/reliability for minimal gain. I remember in NASCAR they used to use special thin qualifying oil and put in thick stuff for the race.

 

[flash7210]

1. Glad to see that no one is berating the OP for using a diesel engine oil
2. We know nothing about the engine build, the car, and how long it's being run on the track for a stint. But it wouldn't hurt to install an oil cooler.
3. Personally I'd go with M1
15w-50

 

[OilUzer]

Isn't 250-290F kind of high and requiring much shorter OCI due to oil break down? Is that the sump temp?
curious why your engine didn't come with an oil cooler.

 

[TiGeo]

Why diesel Rotella with so many great gas Euro 5W40s?

 

[AmBeRnEcTaR]

I don't think it's that simple.

If you use a visc calc, if you plug in 150C, you do not get the same value as is reported for HTHS because KV doesn't present as a high shear scenario.
For example:
M1 5W-40:
KV40: 81cSt
KV100: 13.2cSt
HTHS: probably around 3.8cP
KV150: 5.47cSt
KV170: 4.20cSt

M1 5W-20:
KV40: 46.28
KV100: 8.42
HTHS: 2.6cP
KV150: 3.71cSt

Just using KV, not HTHS, our 5W-40 would have to be 30C hotter (180C) to be the same viscosity.

But, this is of course further complicated by VII content. We could guestimate a ratio. Take KV150 and divide it by HTHS (150).
- If I divide 5.47 by 3.8, I get 1.439
- If I divide 3.71 by 2.6, I get 1.427

Relatively close. We'll say 1.43

Using our 5W-40:
For KV170, 4.20cSt would yield an HTHS 170 of 2.93
For KV180, 3.73cSt would yield an HTHS 180 of 2.61

So, consistent with the 30C that we determined contrasting KV's.

Now, is this accurate more broadly? No idea, but one could test on a few examples and see if it tracks. 30C different is pretty big though. You could also run it the other way to determine the temp that the 5W-20 provided the same HTHS as the 5W-40 at 150C, which ends up being ~123C.

This is so well explained until it !¡ my mind...

 

[Jetronic]

Isn't 250-290F kind of high and requiring much shorter OCI due to oil break down? Is that the sump temp?
curious why your engine didn't come with an oil cooler.

It does require a shorter OCI, by a significant margin even. I'd get rid of the oil after the trackday.

 

[chris719]

I don't think it's that simple.

If you use a visc calc, if you plug in 150C, you do not get the same value as is reported for HTHS because KV doesn't present as a high shear scenario.
For example:
M1 5W-40:
KV40: 81cSt
KV100: 13.2cSt
HTHS: probably around 3.8cP
KV150: 5.47cSt
KV170: 4.20cSt

M1 5W-20:
KV40: 46.28
KV100: 8.42
HTHS: 2.6cP
KV150: 3.71cSt

Just using KV, not HTHS, our 5W-40 would have to be 30C hotter (180C) to be the same viscosity.

But, this is of course further complicated by VII content. We could guestimate a ratio. Take KV150 and divide it by HTHS (150).
- If I divide 5.47 by 3.8, I get 1.439
- If I divide 3.71 by 2.6, I get 1.427

Relatively close. We'll say 1.43

Using our 5W-40:
For KV170, 4.20cSt would yield an HTHS 170 of 2.93
For KV180, 3.73cSt would yield an HTHS 180 of 2.61

So, consistent with the 30C that we determined contrasting KV's.

Now, is this accurate more broadly? No idea, but one could test on a few examples and see if it tracks. 30C different is pretty big though. You could also run it the other way to determine the temp that the 5W-20 provided the same HTHS as the 5W-40 at 150C, which ends up being ~123C.

It’s a bit tricky. It makes me wonder if some high VII oils in a specific grade may work better than you would think in certain parts of the engine because temporary shear keeps the local temperatures down.

 

[ScottyB]

i think if you're tracking the car regularly, one of the first things to address is having an oil cooler installed along with a temp and pressure gauge. from there it might become a lot easier to document and manage your temps and then potentially go up multiple oil grades for added protection without too much of the associated temp rise due to the cooler.

if it was me, I'd consider M1 OW-40 a good jumping-off point just because it almost straddles the line between a 30 and 40 weight. kind of a low risk compromise to see how it does for you. i wouldn't stick with 20 weight at all if the FA20 oil temps run as high on track as i've heard on forums/groups.

 

[TiGeo]

It does require a shorter OCI, by a significant margin even. I'd get rid of the oil after the trackday.

I've tracked my car plenty and don't run home and change the oil after a few days of higher temps. Maybe I'm nuts but I don't think a few hours total of 250-270 oil temps ruins the oil or will cause any issues. My car has an oil cooler that is just a heat exchanger with the coolant circuit....really needs an external one to boost coolling capacity.

 

[Jetronic]

It’s a bit tricky. It makes me wonder if some high VII oils in a specific grade may work better than you would think in certain parts of the engine because temporary shear keeps the local temperatures down.

But places where shear is in issue with regards to temp rise, you want the biggest possible oil film thickness. It's the places that need lubricating.

 

[Jetronic]

I've tracked my car plenty and don't run home and change the oil after a few days of higher temps. Maybe I'm nuts but I don't think a few hours total of 250-270 oil temps ruins the oil or will cause any issues. My car has an oil cooler that is just a heat exchanger with the coolant circuit....really needs an external one to boost coolling capacity.

Going from 250 to 290, the degradation rate quadruples. So at 250 I wouldn't either, but at 290 for a day I'd replace it first convenient time and run that until the next trackday is done.