I am getting ready to do this myself. Can't remember where I've read it, and I have probably been exposed to too much information. But here is what I have read. The universal tractor fluid is a satisfacotory oil, I've also read the type "F" automatic transmission fluid is a satisfactory oil. The goal being finding the oil with the highest viscosity index, which is not the same as viscosity. From what I understand the Viscosity Index is a measure of the fluid's ability to transfer power. I have checked and find the type "F" fluid has the higher index of the oils I have looked up and I plan on replacing the oil in my fluid drive with the type "F" later this month unless someone can scare me off.