Folks: I decided to change the gearbox oil in my 1966 MGB. I have drained the main gear box, but when I attempt to remove the OD plug which is marked "Drain" I can't find the correct wrench to fit.what gives? What size is this? I plan to refill with 20W50 oil.same that I run in the engine.right? Manuals say about 2-2.5 qts. for gearbox and OD? Thanks, Ray Graham