My 318i doesn't work (produce power) any harder than either one of my Accords at the same speeds.
The 318i turns faster to produce the same power, but power output is the measure of the amount of work an engine is doing, not revs.
Waste heat is also a function of how much fuel is consumed, which relates directly to the power output required for a given speed.
What I am trying to say is that if the '99 Accord needs 3000 revs in fifth to produce the power required to go 80 mph, and the 318i does the same at 4000 revs, both must be consuming similar amounts of fuel, and resolving similar amounts of heat into their oil and coolant.
The Accords are geared much taller than the 318i, or course, but must produce pretty close to the same power to produce the same kind of speeds.
Of course, the Accords can get to those speeds more quickly.
They also have a higher top speed than a 318i.
I should also note that our old Civics were geared pretty close to the 318i, and they didn't call for 20W-50s, either.
An alternative explaination is conservative recommendations.
While Honda recommended 5w30 years ago, BMW didn't.
Heck, Honda's BMW killer, the S2000, specs a mere 10w30.
I suspect that BMW's recommendations are very conservative, and are based upon the hope that no driver will be able to kill an engine just by using it hard.
Honda had more confidence in its engines, and therefore recommended a lighter oil.
Would I use anything less than a forty weight in my 318i?
No!
Would I use something heavier than a 5w30 in my Hondas?
Why not?
