I did a little test with motorcycle engine oils.
After a few hard rides, I noticed the oil was quite black. If I drained the oil and let it sit for a week or two, the clutch material would settle out, leaving the oil a lot more clear.
When switching to a lighter oil, I noticed less clouding from clutch material and the oil staying transparent longer.
When testing very heavy lubes in 2-stroke transmissions, the clutches got very hot and the oil blackened quickly.
I'm sure that in 4-stroke engines, the engine would enjoy less clutch material in the oil.
In the above drag race application, I doubt if the engine/transmission oil ever gets over 180F.
At that temperature, an SAE 20 HDMO should have a viscosity of around 11-13cSt.
Perfect for both the clutch and engine at $1.50 a litre.