It is very possible that the Oring is deformed or the filler cap itself. No need to remove the muffler, a major pain and you would then need to replace the muffler gasket. 9 foot lbs is not much torque and very few torque wrenches are accurate a that reading. Put the cap on til it touches and then give it a 1/4 turn. Make sure you do not over fill the final drive. There is not much if any pressure in it.I guess I don't quiet have the 'touch'. The filler cap is on tight now and not leaking. In future, I'll remove the muffler so I can get at the filler cap with a torque wrench and do it according to Hoyle.
Replace the O ring, they do flatten out.