How to run in an engine.
Crank and bottom end run in happens in the first couple of minutes. Keep it running for the first few minutes, very light load or just 'blipping' the throttle.
Top end, rings, cylinder, pistons etc.etc. 90% happens in the first 20 minutes. You can take it easy, you won't damage the engine, but you won't get a good ring/bore seal and the engine performance will suffer. It'll smoke a bit too.
To run in properly, load up the engine in a high gear under full throttle to expand the rings against the bore and 'run in' the wear. Do this for the first half a tank. Ideally, run up a hill under full throttle but only medium revs for say 20-30 seconds, then run back down the hill under no throttle letting the engine do the braking. This will get some oil to the rings. repeat. I'd do this for a tank of gas. You could probably use any mineral based oil and even semi synthetic, but run in and mineral would be best to get the engine 'run in' if you know wht I mean. Once it's been done, switch to a good oil and change regularly. every 5 tanks or so.
I use cheap home brand oil from safeway. My father works in the automotive industry, and believe me, it's the exact same oil as the Shell 20/50 you pay twice the price for. same oil, same factory same every thing. run it in my cars, bikes etc. May have problems with clutch slip, but I hven't so far. They say 10/40 oil for most pit bikes, but in a warm climate like australia, 20/50 would be just fine.
I do this for all my 2 and 4 stroke bikes, racing, road and dirt. Same deal for 2 and 4 strokes. Rotary engines would be a different story I'm sure.
Hope this helps