Do you have the transmission out of the mower? Mine is a 2000 series but probably similar. I take it out, drain the gear box, flush with kerosene, refill with the gear lube through the hole where that vent hose is. Mine gets full of aluminum particles from the ring gear riding on the case and wearing it. I haven't touched the hydraulic unit so far. Transmission works fine but seems to be pretty cheaply made - but at least the differential gear part is separate from the hydraulics.