If I might ask, why the use of such a thick oil in your engine? With tight clearances comes resistance to flow - in other words, where the thick stuff just cannot flow well. 10w-30 should be fine just about all-year round. For the person that likes to run two different types, 10w-40 should be about the thickest and 5w-30 about the thinnest for a 302. No real advantage (more like disadvantage) to running anything heavier or lighter.