Thats why earlier on in the thread it was stated that the oil with the higher rating,for running hot "viscosity", is used at the track where the car cops a continual flogging.
And the lower rating is used for the street.
Ie: a higher rating is impractical for the street,as it will not offer the full protection of the 40w (which heats up quicker).simply because if there were a 60w in there it wouldnt be using the full thermal properties that a higher viscosity rating oil will offer thus the oil would be thicker,in turn inhibiting movement of rotating parts with the thicker oil.which could prolly lead to damage at an extreme.
Thats why synthetic oil rules over mineral.because of it's thermal properties and also its purity.
Correct me if im wrong here.