I use the multi-grade stuff, 75/90. because of the winter temps. Its good all year. Some consider the "EP" additives, not so good for the tranny, and they may have a point. It can cause them to kick out of gear with the helical cut gearing, so they use straight mineral oil. I don't want to have two different lubes, so just use the same in everything, differentials too. If your tranny/T-case is in reasonable condition, you will get by. But I would not recoment the straight gear lube today. Use the multigrade stuff. I never have used the synthetic stuff either, due to cost.