I've had this self-propelled walk behind for about five years and my grandpa a few before that, I think it's about 15 years old. I've never messed with the gear box for the self-propelled. I popped off the vent and was able to put some MaxGear 75w90 in it. Works alot better.. if the belt was tight it would leave burn marks. The problem is though it's leaking a little bit out around the axles. Would actual grease work better? It's a off-brand and the manuals are non-existant.