Interesting subject. I have only seen the inside of a hydrostatic on youtube. But what with the slow inherent wind blast or fan aided cooling it would seem there is some factory concern about heat. So the question: Heat vs additive depleltion in the original oil or heat vs the transmission housing? Or both. My impression is that the hydrostatic is nowhere the complexity of a car auto transmission. We know they should be serviced regularly, including filter. Enthusiast car buffs change out manual transmission oil regularly though not specified by factory. I have run several manual car 5 speeds 100K with no changeout, Others seemed to benefit from changes where synchro rings needed help for smoother shifting. I feel my own Craftsman DYT 4000 is slipping a little when cold. Please respond, knowledgeable forum people!