pitmaster gave good advice. Don't get too hi tech about the oil choice. Simple rules:
1) Change it often. The clutch particle and metal contaminants cause more problems with oil flow than the oil getting old. I replace mine after about every 2 or 3 hrs.
2) Use 15w40 in the winter and most of the time in Aus. But consider 20w50 in really hot temps.
3) Get a magnetic oil drain plug. You'll be amazed!
4) Don't bother with expensive oil. These china engines lose so much metal (off gears etc) that you have to dump the oil often (see point 1). Why throw away expensive oil so often.
5) Be green: Your local servo should take you used oil for free (they sell it).
My oil: I use Shell Rimula-X 15w40 diesel oil in my mini. 100%. Great for wet clutches. Great oil. $60 for 10L of it. I use it in my 450F too, and I get near zero engine wear in that engine.