I promise I don't read all the threads. I just clicked on this one because I have this bike and I can't sleep.
My personal take on what happened with the Cub (I know it's 4 months ago but I miss posting like this, ), Cubs (50, 70 and 90cc) can not physically be overfilled with oil as the dipstick is very low on the case and leaks oil out the side if you do overfill. It's done that way on purpose. This doesn't apply to any other bike however. If the bike was smoking a lot, I suspect something else is wrong. Without seeing the bike I would say oil scraper rings, and overfilling just caused more oil to "splash through" into the combustion chamber. It's likely worn already though and on it's way out.
FWIW, I have an FI Cub as of now (back home), and I "overfill" it for extra cooling and last oil change (2000 km, I was very late on it) netted no drop in level on the dipstick. But re: overfilling, lots of "us" Cub owners do that when heat is a problem such as when we race or spec up our motors, we fill up to a litre vs. 600ml. My 68' and 2001 are also filled to a litre, none of them use oil (practically speaking) or smoke either.
Now onto oil control/burning. I'm just gearing this towards Cubs.
To get an engine to not "use" a lot of oil, very specific tolerances and finishes have to be met on all sealing surfaces. This takes more skill than not to achieve, and a bit of maths and planning ahead, all stuff I have not really found plentiful in Cambodia. That's why I either did my own machining (as with your bike YTP) or sent out the stuff to Vietnam. I have had very bad luck with Khmer doing the machine work, even for my old Daelims.
Cubs usually bed in around 1,000 miles from new, but (and this is where it gets tricky) depending on the skill of the machinist and the quality of the machinery/tips, some bikes can bed in as little as 50 miles, and sometimes if done wrong, they'll never bed it at all. It's all about getting the desired surface which balances metal to metal sealing, and appropriate lubrication to the rings at the same time (the 45 degree crosshatch).
Misc, Cubs also (usually) don't have their crankcases vented to the airbox, they're dirty bastards and dump it on the road.
So where am I going with this? No idea, but anyway in terms of oil burning, true any motor will always burn oil regardless, but it is possible to get a motor to use very little oil to where the loss of oil is negligible to the total capacity. Most cars nowadays are at this level of technology, so much so to the point where most people consider any oil loss now a sign of poor workmanship and quality.