The model posted has a General Transmissions RS800P. This a CVT transaxle and not a Hydrostatic transaxle.
First there is no oil in this CVT transaxle it contain grease. Also there maybe an inversion control problem where it not going into gear fully. It may only need adjusting due wear of the plastic parts.
Only external control components are available if the problem is internal then it is a new transaxle time.