It is possible that opening the choke before it burps will prevent it becoming over-rich, but I don't think the normal procedure (waiting for the burp) would prevent it from starting unless it had a fault such as a bad spark plug. You can find out though: after the engine has been run, let it get completely cold, set full choke with ignition on and throttle set to the start position (that is, the detent), pull it three or at most four times, and if it doesn't burp, immediately remove the spark plug and see if it is wet. In the normal course of events, if the ignition system works and the spark plug is good, you can't get to a too-rich situation without passing through a more-or-less-right situation on the way there, and it should have burped at that point. If the plug is wet, you most likely have a spark problem.
With regard to fuel and oil, I suggest you use the octane level recommended by Stihl. Standard unleaded is 91 octane and super unleaded is 95 octane. So far as oil is concerned, use a brand-name 2-stroke oil, in the proportions recommended in the Stihl owner manual.
