Engine oil 15w50
 

A friend shared the following about motor oil. Some of you already know most of this, but I thought it would be a handy reference for the Tech Chat forum. Disclaimer, this does not represent opinion or sanction of SOA:

============================================
I am a mechanical engineer , so I know something of lubrication technology and bearings. I am, however, not an automotive engineer, so what I say has to be taken with a grain of salt.

The logic goes like this: Wrist pin, crank, and rod bearings bear the load from combustion chamber pressures and intertial forces.

The combustion forces go up linearly (at least) with engine output. There is a term, BMEP (Brake Mean Effective Pressure) which expresses the average cylinder pressure required to produce the horsepower and torque. Double the torque at a given rpm, and the BMEP has to be doubled at that point. Double the torque at a given rpm, and the horsepower doubles. The WRX produces around 113 hp/L (very high) and hence very high BMEP; consequently high bearing loads.

Bearing loads due to inertial forces increase with the SQUARE of the rpm. If you run consistently at high rpms, average bearing forces are higher.

So driving your WRX hard, making a lot of power (high BMEP) at high rpms really loads the bearings. Loads on the bearings are resisted by the oil film (which is partly hydrostatic (from oil pressure) and hydrodynamic (like hydroplaning on a wet road). Low viscosity oil has thinner films and less ability to resist these bearing loads. Oil temperature also goes up during hard usage, further reducing viscosity and oil film thickness.

So higher viscosity is good in hard usage situations. There are also reasons not to use high viscosity:

- Worse fuel economy
- Poor flow during cold start (when a lot of wear occurs)
- Small oil passages in some modern engines may not allow enough flow and cooling with high viscosity oils.
- Some hydraulic valve lifters are designed for low viscosity oils and don't operate properly with high viscosity

Given that Subaru states in the manual that up to 20W50 is permissable in the case of trailer towing and other hard use, we don't have to worry about warranty issues or the oil passages being too small for higer viscosity oils. The WRX has solid lifters, so no problem there. Sythetic oils flow much better at low tempertures than equivalent viscosity mineral oils, so cold start lubrication should be OK. And the synthetics are MUCH better at resisting high temperature breakdown (as in the turbo bearing at high sustained boost).

Net is, I conclude that M1 15W50 is a good choice for someone that actually uses their WRX. Mobil says that it is OK to switch grades seasonally, as the basic oil chemistry is the same across their viscosity grades. So one could use 5W30, 10W30, or 0W40 for normal use in very cold weather (like parked outside at 0-10 degrees, etc.).

Sounds pretty much inline with what I learned in my IC engines class in school.

Re: Engine oil 15w50
 

That's what's in my baby. I started out with 5W30, but was worried it wasn't affording me enough protection as I started upping the power of my motor, so I figure the loss of power from the heavier oil is worth it for the increased lubrication.

And yes, the 15W50 does effect the cold start. I gotta crank it a bit on cold mornings, and the idle is a little sloppy... 'course, I think the under-drive pulley adds some slop to the idle.

Re: Engine oil 15w50
 

say goodbye to mister warranty. if you pull into a subaru shop and they drain the oil out.... bye bye! just my thoughts.

I was under the impression that 15W50 is withing the specs in the manual.

There should be no warranty problem with 15W50. The generic Impreza owners manual recommends 5W30, 10W30 for higher minimum temperatures. These are to display the Energy Saving logo. Severe duty (such a trailer towing) allows for up to 20W50. Mobil 1 15W50 fits within the viscosity/temperature ranges listed in the manual. It is, however, not an energy saving oil. (As if staying in boost is energy saving!!).

SO, If I was to purchase a WRX/STi, would it be better to run conventional OILS vs Synthetic ????
Is this a warantee issue ??

Most people recommend running conventional "Dinosaur" oil for the first 5000 miles (or at least until the break-in period is over) since it'll help break-in the motor. Then switch to Synthetic.

I used dino-oil for the 1st 4000 miles, then switched to Mobile 1 during my 2nd oil change. (Changed it at 1000 miles when the break-in was over, then again 3000 miles later)

I have been using 5W30 since 3000 miles... would it be better to convert to 15w50 now with my hard driving habits?

15w50's a little thick in the winter... I run it when I'm autocrossing for a little added protection at high rpms. But if I know I'm going to be driving long distances, I'll put in the 0w30 (i think that's the normal Mobil 1 stuff)

I switched to 15w50 and it seems to run a a ton better in the hotter weather.

The tech guys at SOA headquarters in New Jersey suggested that we use 20W50 in our race engine (which is internally stock). We only use Redline 20W50.

I don't think I would ever use a 0W30 unless I expected to be rebuilding much more often.

Gary
Sheehan Motor Racing
www.teamSMR.com

Where can you pick that up Gary?

I get it at my local Monument Car Parts.

I don't think I would use it in a street car under warranty, though. As far as I know, it is not API certified and may void your warranty. Yes, even though it is far superior than certified dino lube.

Gary
Sheehan Motor Racing
www.teamSMR.com

The Mobil 1 0w30 (or it might be 5w30 I'm drawing a blank) is what Mobil 1 recommends for motors that are supposed to have 10w30. I guess their formula is supposed to protect so well that you can get away with a lighter oil, and still have good protection.