ATF works well and is recommended in the FSM for cooler climates. They shift real nice with it. I have used it year round with no issues. Don't run anything synthetic unless the four speed has had syncro rings designed for synthetic lube installed. They have extra chanells cut on the inner diameter taper to help bleed the lube out when shifting (synthetic is so "slippery" that the syncros don't work as well). All new syncros that I have seen have this done to them. Fill it until it just dribbles out the filler.