John, I have sent you a pm regarding operators manual.
Gearbox oil should be a non EP grade,( ie GL4 not GL5), sae 140, I use Morris Oils AG140
To drain the gearbox oil there is a drain plug located towards the front of the right hand half of the gearbox casing, near the join between the two halves of the gearbox case. DO NOT attempt to undo the bolt towards the rear as this holds the gearbox oil pump in place.
Fill gearbox through the hole for long dipstick, operators manual details how to check levels. The level on the short dipstick can only be checked after running machine in neutral for 2 minutes as the oil is pumped up by the oil pump.