Engine oil recommendations for turbo cars
 

What weight and brand oil would the experts recommend for a turbocharged FRS? It's a stock bottom end, boosted to ~300HP. I'm running a Skunk2 racing oil cooler setup, but am mindful of increased temps given I'll be using the car on track. Mileage will be 50% track / 50% street getting to the track.

I'm in Canada, but the car will never see winter use.


I'm thinking I should run something with a bit more weight at high temps then the stock 0W-20 fill - especially to help maintain acceptable oil pressure at idle once hot.


Would a full synthetic like Amsoil or Royal Purple in 15W50 work? Or do the oil passages/clearances in the stock motor require a thinner oil?

stock engine? stock weights.

I agree with Mike. These are not EJ motors, stay stock or a 5w-30 if you feel compelled to be racecar. You've got an oil cooler so you should be OK. If you're really concerned with oil pressure, look into an accusump

mobil 1 makes a 0w-30 for what its worth :iono: but idk much

I'm asking about oil weight because I had read about ppl having trouble with oil pressure under 2 situations with these cars, once the oil was hot:

1) Difficulty maintaining at least 10psi oil pressure at idle; and

2) pressure dropping at high RPM on track.


Are these not problems with these cars? And are they not exasperated when you're running a turbo?


What I don't know, is what kind of oil temps these guys were running when experiencing those problems.



So - what is the safe temp range? And at what temp should I be immediately pulling into the pits?

I will be running full synthetic - either Mobil 1 or Royal Purple.

If your pil cooler can keep oil temps below about 115C you should be ok . With low viscosity oils

Most guys with FI usually run 5w30 or 0w30 or 5w40 0w40

Mobil 1 5w-30. Ran it for 3+ years boosted of hard driving on a hot climate. I wouldn't get caught up in all the amazing (and extra expensive) oils out there unless you plan to track it heavily.

The answer is the thinnest possible oil that gives sufficient oil pressure. Get an oil pressure gauge.

This is a scatter plot of my car of oil pressures (Y axis) vs revs (X axis) where the colour is oil temp. The data is a shakedown run on my turbo setup, maybe 5 or 6 laps with just two hot laps. Stock bottom end, 10W-40 synthetic oil, 240rwkW on e85 with an oil cooler.

As you can see oil pressure drops away steeply as oil temps increase. The rule of thumb is you want 10psi of oil pressure per 1000 rpm. E.g. 70psi at 7000rpm. The more power the engine makes the more oil pressure you need.

What this data tells me is 10W-40 is barely thick enough. The cross is showing sample of 52.9psi at 7335rpm with oil temps of 115C. This is too low a pressure.

I think an xw-30 will be too thin at elevated temperatures on a track after a few laps. A 40 weight is the minimum on a stock bottom end. I'd have no hesitation running a 50 weight if you were happy to do an oil change before and after track days as you mentioned.

An oil cooler and oil pressure gauge are both essential items if you are tracking this with a turbo.

Thanks, Hags for your very thorough reply! May I ask what brand/line of 0W-40 you were running?

I've been looking up specs on various oils, focusing mainly on the Viscosity numbers @ 100 deg C and @ 40 deg C. What I'm finding is there is quite a variance brand-to-brand within a given spec'd oil weight.


My thinking is that if 2 oils have a comparable viscosity at 40 deg C (meaning they will pump similarly when 'cold') the one with the higher viscosity at 100 deg C would maintain pressure better as temps increase.


I already have an oil cooler and oil temp gauge, but will now look at installing a pressure gauge as well. Finding a place to put it could be a bit of a challenge, though!

It is Penrite 10 Tenths Racing 10W-40 (I know I said 0W-40 in the post but when I went to check it was actually a 10W-40. I have edited my post to reflect that.

Maybe there is differences between brands with the same rating. I've never looked. The first number is the cold temp but this is below freezing temps, not at 40C. The second number is the viscosity at 100C. So a 10W40, 0W40 or a 20W40 should be the same viscosity at 100C.

The place to learn about this stuff is the Bob the Oil Guy "motor oil university" page

https://www.bobistheoilguy.com/motor-oil-101/

OEM turbocharged Subarus use 5w30--not 0w20. Why would 5w30 not be the minimum? Then he could use thicker oil if there are serious track days.

https://www.patriotsubaru.com/synthetic-oil.htm

I really like this logic. OEM's are forced to make so many compromises with their recommendations and design it is impossible to get one recommendation for everyone.

A 0W-20 is great for a car that isn't tracked, doesn't have a turbo and the associated heat it generates, needs to protect the engine when starting the car and driving 1km to the shops to buy some milk, whilst optimising fuel economy. 0W-20 might also be just OK for a NA car doing a hot lap or two in the overall balance of "best oil".

It's a hard compromise for the Toyota and Subaru engineers to balance all these requirements. But that doesn't mean follow the OEM book when you have a turbo and are driving hard for extended periods.

Interestingly the GT86 OEM Toyota race series in Australia a lot of the competitors use 0W20. That is because it is a fixed spec race series (oil weight is free spec) and very thin 0W20 sucks less engine power that pumping a 50w. Those extra few HP are whats needed to win and they just accept the fact they will destroy engines doing it. If you are in a competitive form of motorsport where 5HP is the difference between winning and not, then protect the engine and make sure there is enough pressure when the oil is hot.

I only suggest not going thick because you're asking about the winter grade. Your oil is still lubrication on a cold start. A 5 winter oil is thicker when cold than a 0. When cold. Cold. When your oil heats up, it has the viscosity of an SAE grade 30 oil were it a 0w30 or a 5w30.

If the turbo doesn't affect your cold temps (which it doesn't) you wouldn't want to make cold oil thicker right?

**EDIT**

just realized you changed the 20 to a 30. That's the only place you'll find merit is increasing the warm grade. A 0w-30 would be ideal..

Still, and yet, Subaru recommends 5w30 on their turbo engines. Why do you think that is? I mean really, why?! I don't know. Is it because 0w30 is hard to find or is more expensive?

Well, to be frank, adding a turbo does not change the clearances in the block or other things they may have fine tuned with our unique high compression fa20 block. Could really just be because Toyota had a say and wanted better emissions, it's likely something far less worth our speculation than we could imagine. As I had said, the FA20 was far different than the EJ motors and maybe they just decided to try a light oil

The FA20DIT in the Forester XT and WRX and the EJ in the STI run 5w30. The newer (?) fb20dit (?) runs 5w30. Every NA engine in their current lineup like the Crosstrek, Imprezza, etc including the BRZ run 0w20.

The increased torque negatively impacts the design oil film thickness.

all turbo subarus run 5w30 all NA subarus run 0w20

Huh. Yeah I don't pay attention to Subarus, wasn't aware of the trend. Guess you've come to your own conclusion then :)

I've read (but I can't remember where) that oils tailored to perform better at temperature extremes break down faster. ie That a 0W-50 oil fill breaks down to be something more like 10W-30 pretty quickly due to long-chain hydrocarbons shearing.

So I wonder if Subaru's thinking is along these lines - that a 5W-30 has greater longevity than a 0W-30?

This graph is quite interesting.

One thing I see is that in addition to the pressures at high RPM/high temp being too low (under the target of 10psi/1K RPM) the pressures at lower RPMs with lower oil temps seem to be excessively high.

From what I've read, my guess is that dropping to a 0W-40 oil (that has a comparable 100 deg C viscosity) will drop the low temp data points to a more acceptable level.


I'm also a bit shocked that pressures were recorded in the 75-90 psi range. I figured the stock oiling system would have a bypass valve that limited excess pressures, but perhaps it doesn't?


In reality, these excess low temp pressures (on their own) would likely only be a concern on a street-driven or dual-use car. And even that could be mitigated by letting the car thoroughly warm up before driving.

HOWEVER, changing oil weight to drop these pressures could also work in concert with a new gear/oil pump drive designed to provide greater oil flow, and therefore oil pressure.

Hmmmm.

The previous data was on a track with a min oil temp of 90C or so.

This data is some daily driving. The scales of the pressure and colour have changed as oil temps here are a minimum of 17C. Lots more pressure. I wouldn't worry about too much pressure. Seeing 200psi on a cold morning isn't uncommon.

I'm having an engine built now and they'll modify the oil pressure relief valve to hold more pressure before it opens.

Do you mean modify it to have it hold more pressure or less? I do not think the relief valve opening is limiting your pressure on track.

My thinking is that having the relief valve open SOONER would allow you to run -50 or -60 weight oil, boosting high-temp high-RPM oil pressures while limiting pressures at the colder temps. It would essentially help "flatten" the pressure vs RPM curve.


In the reading I've done the last few days I did come across articles that talked about excess oil pressure 'washing' out the bearing surface.

Shimming the relief valve makes the spring stiffer, keeps the relief valve closed longer. Holds more pressure. You can see how the pressure raises nicely with RPM and then flattens out. Stiffening this spring should move that transition to a higher perssure/rpm point.

The 10psi per 1000rpm is a rule of thumb for minimum pressure, not a target. High pressure is not a problem.

At cold temps those thick oils don't flow and allow engine wear. Even with a lower pressure on the relief valve. You want the thinnest oil you can that holds sufficient pressure.

My engine will be finished in two days and back in the car within a week. I guess we'll know for sure then.

Looking forward to seeing the results !