I still don't understand how oil leaking into the exhaust port wouldn't produce visible smoking from the silencers, but you and the OP have both provided "testimony" supporting this. I also don't understand why the OP's leak, expressed on the exhaust nut, didn't start a fire.
Years ago I made arrangements to have Leo Goff freshen up a Mk3 head. I told him that I had the Norton valve guide tool and that I would save him some time and punch out the guides. Absolutely not he said, we machine them out and measure/machine the new guides for the factory specified fit, for each hole.
Thanks for the correction to you and the OP.
Best.