I recently drained and refilled the gearbox in my BT7, including the overdrive. Oil was pretty dirty, and since the car is a recent purchase, I'm thinking it might never have been changed. I put in Amsoil GL4 75W80 gearbox oil, supposedly close to SAE 30 engine oil in terms of viscosity and recommended (I thought). My OD worked perfectly prior to changing the oil, now it doesn't work. Relay and solenoid work, but not the OD. Looking for suggestions on what might be the problem.