With my limited experience, i find if i run too much oil in the gas, i will foul out plugs and sacrafice a little bit hp. Spark plugs are cheap and i dont race for pinks so more lubrication and foulign plus is better to me than rebuilding a top end often.
When i got my shee, shee was running 24:1 of benoil i believe. Once i ran the tank out, i mixed some amsoil 32:1 w/ 93 octane and it's never fouled a plug and it runs like a champ.
I find my yz runs rich though with 32:1. I dont know why. I think i might try some 40:1 in it. Kinda hard when you only have one gas contrainer. Ah well, ill just mix 1.5 gallons and dump it in.