I cast my vote for the classic stand-by lawnmower oil, straight 30W.
Synthetic oil in a one-lunged non-racing clone engine is a waste of cash. However, if that’s what you have laying around in quantity, it’s fine to use of course. But any quality dino oil (non synthetic) is all you need, just be sure to change it often per manufacturer recommendations because as it gets dirtier and thicker, it no longer gets distributed to the vital parts inside the engine (by the oil slinger) as well as it did when new.
I’ve cracked open a few abused engines that, yes indeed had oil in them, but it had long turned to sludge and the crankshaft journal, cylinder sleeve, etc was scored to hell as a result of oil starvation.
Rotella T6 is the exact oil I use in my turbo Saabie, but I refrain from using it in anything else as it is designed for automotive use along with a filter. You’re not gonna see the same benefits in an non-filtered, splash-lubricated minibike motor.... IMHO