The numbers are indexed values of viscousity classified at set temperatures.
The lower one at 20degrees celcius(68F) I think and the higher is at 100 degree celsius (212).
All non M-models come with Castrol SLX(?) 0w-30 here.
BMW recomends Castrol TWS 10w-60 for M-models. And that is what I use.
Low cold viscousity index is pretty much allways good. Makes sure you allways get lubrication from start-up. And it provides lower friction in the engine.
You want the warm viscousity index high if you drive the car hard and get high oil temps.
Race oils are usually 20w-60, they usually warm along time on idel/low rpms so the cold index isn't that important.
The wider the spread the more expensive it is to produce.
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1994 Arcticsilber M3, 286hp, 18" AC Schnitzer wheels.