Having difficulty with your picture. Some sites won't link to the board. However,
The level plug is a square headed plug near the PTO lever. Fill at the rear of the seat until oil just flows out of it.
There is a similar plug on the side of the gearbox and the filler is near the gear sticks.
Oil is 30W or 20/30W or 15/40W. what ever you use in your engine for simplicity.
The filter is the canister to the left of the drive shaft in the picture, the silver filter, on the right, is the returns oil filter
If this is the valve you have, or similar, yes, pulling the knob diverts the oil from the pump to the trailer pipe.