Heat would be the enemy here. It could break down the viscosity. The other thing is wear. Little micro size pieces of metal. I think changes at 100k would be good as long as it is a complete drain and flush or you didn't accomplish much. Final thing is have the fluid you want to check sent to the lab. We did on the big rigs. Gives you an idea of wear items like bearings. If it comes back good no change needed.