I recently changed my gearbox oil.
You will need 1.6L of Honda mtf oil. This comes in 1L bottles. The drain plug is located close to the driveshaft & the fill plug above it. Make sure you can undo the fill plug before removing the drain plug or you will be in big trouble!
The crush washers are different sizes. The drain plug is the same size as the engine oil drian plug. The fill plug is larger at 28mm O/D. I used a 5/8" PVC hose along with a 5/8" 90 degree barbed angle to refill. I found it easier to remove the battery & tray to allow easier access to insrt the fill pipe from above after faffing about trying to insert it from below. There are youtube videos to help.
Fill plug Crush Washer is 28mm O/D, 20mm I/D & 2mm thick.
Honda part number: H9410920000. £0.85 each.
Drain plug crush washer 22mm O/D 14mm I/D X 2mm.
Changing the oil has made a noticable difference to the gearchange. I can almost flick between gears with my little finger now. So, I would recommend changing it.