My 2001 Dodge WS-1500 pickup will be ready for a transmission oil change (5 speed manual) soon. The Amsoil rep said to use either diesel 5W-30 or ATF in this transmission. I live in Richmond, VA, so the climate is fairly warm. It tows a 2000 pound boat, but that's the heaviest use it sees. Which oil would be a better choice for smooth shifts, or should I just refill with the GM Synchromesh type fluid? Does anyone know how many quarts this transmission takes? Thanks.