Sorry to be a bugbear here but, it shouldn't be running great, it should be a bast4rd to start and the plug should be as oily as the tailpipe, worse even as engine oil (i don't mean 2stroke) isn't designed to burn. Also it does smell foul believe me, really acrid taste at the back of the throat taste.
Its the plug thats making me wonder if you have 500 miles of sludge slowly burning off the pipe, my TDR did it recently and was like a red arrow for a day or two.
How much oil has it actually been through, the way you are describing it it will clear out the gearbox in short order, its only about 70cc between max and min that would be gone in a couple of miles, they do use a little and those little windows are a pain to get good levels from, even the dipstic for mine (and no i don't mean me) isn't perfect. (well maybe its me)
I'm happy to be proved wrong but it isn't shouting crank seals at me.