The AD2 came fitted both with and without a reduction gear-which do you have?
Mine is direct drive- no reduction gear, and the transmission is lubricated by the motor
oil (which must be SAE 30 straight weight oil).
Those with a reduction gear are lubricated independent of the motor
oil, and have a second dipstick on the transmission.
I have the shop manual for this motor, and some other materials describing the adjustment and troubleshooting process for the transmission. I'll get these together and see what they say.
I assume you've checked the fluid levels and things are normal?
Don't give up hope- these are solid, simple motors and trannies.