Well said silver
I would jus' like to add that ANY oil, synthetic or otherwise will be prone to undue carbon build up and deposits if used far in excess of the manufacturer's recommendation.
85-90:1 with a 100:1 synthetic or 40:1 w/a 50:1 synthetic is about as much as I'd push it and even then only during break in and/or you notice excessive heat. Something crazed like 32:1 with a 100 or even 50:1 synthetic is jus' beggin' for goo gummin' up the works
Followin' the (oil) manufacturer's recommendation is yer best bet and remember - yer spark plug tells no lies