I have just bought a nice 2000 Express LS passenger van with a 5.7L engine and a 4L60E transmission to use as a work van. I have a couple of questions: Anyone run full synthetic engine oil in their Express? If yes which brand and why? Engine oil seems to have been changed every 3000 miles with Dino oil. Also I notice a slightly rougher shift when my transmission is hot. Fluid looks nice and clean and does not smell burnt. Thinking Synthetic ATF might help? Royal Purple? Is synthetic AFT worth the $80 cost? or should I just replace the filter and refill with Dex/Merc? Anyone fitted an AFT cooler to this transmission? Worth the cost? I do not tow but carry 800lbs of tools daily. Thanks for all your help!