Can someone explain why the engine uses heavier weight oil?

[Ateam]

I am pretty naive about oil and am wondering why an engine with high speed moving parts such as the last few M engines in the M3 and m5/6 require the 10/60 grade. Meaning what exactly does the thicker oil do that thinner oil does not? I would think a thinner oil would be easier to move and thus better lubrication so I am not following the logic to this but its obvious that my logic is off base!

[M3er]

Originally Posted by Ateam

I would think a thinner oil would be easier to move and thus better lubrication so I am not following the logic to this but its obvious that my logic is off base!

on the other hand, thicker oil, vein thicker, can be considered as more 'protective' ...
but still I don't know the real answer on this...

[mdosu]

http://www.speedtv.com/forums/viewthread/471026/P10/

interesting stuff here.

[jphughan]

Read about various oil weights and viscosities, it's actually pretty interesting -- especially how they achieved variable viscosities rather than fixed weight oil. But to answer your question, thin oil is great for low temps because heavy oil at cold temps won't move properly, which means the engine won't get sufficiently lubricated. But as RPMs and temp increase, the lower viscosity isn't needed to get the oil to circulate properly, but thin oil will start to break down, hence the need for thicker oil that can withstand that type of duty cycle while still moving sufficiently well.

For further reading: http://www.upmpg.com/tech_articles/motoroil_viscosity/

[Ateam]

Originally Posted by jphughan

Read about various oil weights and viscosities, it's actually pretty interesting -- especially how they achieved variable viscosities rather than fixed weight oil. But to answer your question, thin oil is great for low temps because heavy oil at cold temps won't move properly, which means the engine won't get sufficiently lubricated. But as RPMs and temp increase, the lower viscosity isn't needed to get the oil to circulate properly, but thin oil will start to break down, hence the need for thicker oil that can withstand that type of duty cycle while still moving sufficiently well.

For further reading: http://www.upmpg.com/tech_articles/motoroil_viscosity/

Thanks but I understand the need for two grade oil and the differences between hot and cold engine temperatures effecting viscosity. My question is why the m3 needs such a thicker oil than almost any other engine out there. A 60 grade is so much thicker than a 30 grade which almost all cars use now adays, including the e39 M5 and the new turbo M's.

[jphughan]

Originally Posted by Ateam

Thanks but I understand the need for two grade oil and the differences between hot and cold engine temperatures effecting viscosity. My question is why the m3 needs such a thicker oil than almost any other engine out there. A 60 grade is so much thicker than a 30 grade which almost all cars use now adays, including the e39 M5 and the new turbo M's.

Interesting, the E39 M5 originally specified 10W-60 and it was changed to 5W-30 later, but then they went back to 10W-60 on the E60. I don't know enough about oil to know why they need such a heavier weight than other cars, but I have to imagine it has to do with redline and the temps that come with all that kinetic energy.

[Ateam]

The e39 m5 engine ran to 7k and was really no different in terms of characterisitcs than the V8 in the non-m e39's.

I guess it must come down the high revving engines must create more piston speed and thus more potential friction and heat production. I guess a higher visocosity oil will survive better and continue to be "thick enough" to lubricate where as a thinner oil would probably get too thin. So my guess is a 5w30 gets thinner as the engine warms up but never too thin to protect the parts. The 5w30 probably would get too thin at the temps found in the oil of the s65.

Guess that is my understanding.

One thing I learned is that there are more opinions about oil grades and multiweight oils and more misconceptions on the net than there is between the power v. torque arguement!

One thing I learned from spending an hour reading the net was that many(including me) had the misconception that a 5w30 had a viscosity of 5 at low temps and a viscosity of 30 at higher temps. I thought the oil got thicker as it warmed up due to some magic science.

Although intuitively and what actually happens is the oil (regardless of grade) gets thinner as it heats up. So a 5w30 is still thicker cold than hot. However when hot, it only gets as thin as a 30 weight oil. v a 10w60 that when the oil gets hot, it gets as thin as a 60. So at temperature the 60 is thicker than the 30 but both types still are much thinner at temperature than at cold start which I guess makes sense.

Confusing stuff

[Leonardo629]

I forgot where I read this, but n/a M engines are built "loose" by design, and this means free and faster revving, but this also means thicker oil is needed to prevent blow-by.

The factory 10W60 TWS oil gets sheered down to a very light 50wt oil after just few thousand miles of usage anyways, and I actually experimented with Motul 300V 10W40 oil when I had my Z4M...and oil analysis from Blackstone Lab says as long as I change every 6000~7000miles, I would be okay.

FYI, X6M/X5M calls for the Castrol 5W30 BMW oil....

[FLOSS M]

new M5 will run on 5W30, not 10w60

[M3Bahn]

They had a engine bearing recall on the early E46 M3s, it was then that they switched to the 10w60 oil.
Bearings float on a film of oil, thicker oil equals better film protection.

[alpina527]

Lets not forget the problems that BMW will have in recommending an oil suitable for all users.

In reality, some M owners who drive short distances and don't go to the red line at every opportunity will need a different oil to someone who regularly tracks their car in hot climates.

We would all be complaining if we were being told to tailor the oil to our driving habits.

The 10w60 is just a good compromise

[JCtx]

Originally Posted by Ateam

I am pretty naive about oil and am wondering why an engine with high speed moving parts such as the last few M engines in the M3 and m5/6 require the 10/60 grade.

There's no logical reason for it. It was probably spec'ed to mask a design issue. Much more expensive engines that rev higher than our S65 use 5/40. Every Ferrari, Lamborghini, and Porsche engine spec 5/40. And only Lamborghini recommends 10/60 ONLY FOR TRACK USE. No such recommendation from Ferrari or Porsche.

My guess is lousy tolerances. That explains main bearing failures, ridiculous oil consumption on some cars while none on others (mine ), and mild to super noisy valvetrain noises as well.
Problem with 10/60 is it's too thick for start up (especially in winter), meaning precious seconds more without proper lubrication. This is a long-term issue, so very hard to quantify or prove. I'm still using it for warranty reasons, but for 90%+ of owners, such oil makes no sense, especially in freezing climates. Bottom line is we'll never know why BMW spec'ed it... but we can certainly speculate . Good day.

[-=Hot|Ice=-]

I think BMW put in 10w-60 because they knew that some of the cars may be tracked, so they put in 10w-60 to stop any 'My engine blew up' claims.

[PhillyNate]

Originally Posted by elp_jc

There's no logical reason for it. It was probably spec'ed to mask a design issue. Much more expensive engines that rev higher than our S65 use 5/40. Every Ferrari, Lamborghini, and Porsche engine spec 5/40. And only Lamborghini recommends 10/60 ONLY FOR TRACK USE. No such recommendation from Ferrari or Porsche.

My guess is lousy tolerances, mostly bearing ones... as evidences by several such failures, plus cars with no oil consumption (mine ) to others gulping oil . Problem with 10/60 is it's too thick for start up, meaning precious seconds more without proper lubrication, making matters worse in the long run. I'm still using it for warranty reasons, but for 90%+ of owners, such oil makes no sense, especially in freezing climates. Bottom line is we'll never know why BMW spec'ed it... but we can certainly speculate . Good day.

My car is kept in a garage that rarely falls below 50 degrees. Heavier oil circulation shouldn't be much of a problem for me right?

[JCtx]

Originally Posted by PhillyNate

My car is kept in a garage that rarely falls below 50 degrees. Heavier oil circulation shouldn't be much of a problem for me right?

Not at all, so no worries. I'm on the same boat, except when I use the car on winter trips. Had to start it once at around 20ºF. Probably took out 5K miles out of my engine's life . Put M1 5/40 (ext. perf, also group IV) and TWS 10/60 on the fridge (32ºF), and the difference was dramatic .

I use the car very little, and live in TX, so am not too worried about that . But if I lived up north, and used it daily, I'd have changed to 5/40 already, especially since I don't track my car. Another very important consideration is NOT to start the engine unless you're going to drive at least 10 miles in summer (or 15 in winter), to evaporate all condensation from the engine. The battery should be charged with a charger, not by starting the engine. Take care.