I am curious what others think. My jetboat has a 460 Ford that is almost always running between 3500-5000 rpm's, only run in 77-93 degree weather, and see's 80-120 hours usage per year (summer only in rainy Seattle). I have used straight 40w Valvoline for years, but last year switched to Royal Purple 40w synth (which gave almost 2 mph increase, gps). I always cruise at 1500-1800 and let engine temp come up to around 150-160 before the flogging starts. Is my thinking correct for using a straight 40w for this application?