My RV has an open diff in it's Dana 70, and it was disgusting from possibly the original oil. At first I put in conventional SuperTech 80w90 gear oil, then changed it to conventional SuperTech 85w140 after a couple thousand miles of mostly highway driving. Next time, I will probably put in full synthetic Valvoline 75w140 since the gears should be pretty cleaned up after 2 oil changes like that. I have a reusable LubeLocker gasket so it makes it easy to change the oil. I don't have to clean off RTV and gasket material from the mating surfaces.