I know they shouldn't be related, well, perhaps if you ran 20w-50 in a car requiring 5w-20. My car has electric cooling fans that don't turn on till 215 degrees. 5w-30 penzoil made my car run right at 200-205, even while idling in stop and go traffic. Whenever I switch to Castrol 10w-30, my car heats up faster, not oil temperature, but coolent temperature. Well, it seems any 10w-30! Cars and trucks with belt driven fans probably wouldn't notice a difference, but does anyone else notice a difference like this between different brands of oil or viscosities?