First is to know the size you need-----Having lived/ridden 25 years in Germany, the Germans love crush washers! You ought to see the number of washers in a air cooled BMW Bike oil change kit....for whole bike I think the oil drain plug on my XTown 300i is flat washer, I just re-use it like the Japanese do.
With most Likes ridden extreme low mileage (Under 10,000 a year) I would not bother to change rear end oil, for a least 2-3 years....but of course check lube level not and then