I'm not sure where the cut off is but 12% should be fine. I did ask a friend at a hobby shop to order fuel in for me (I'm rural so I buy bulk). I wanted 10% Synthetic/5% castor oil. Anyway 50 Ltrs of 18% oil turns up. This was higher then I wanted but thought it would be ok. WRONG. Had problems with tune, plug fowling up etc. After trying many things finally remixed the fuel to 11% (not sure what % is syn/castor) and everythings sweet!

Two meets ago I left the car idleing outside the toilets b4 the final while I took my little girl to the little girls room and when I returned after awhile shes still running, a few blips and she was off. With the 18% it would of choked for sure.

Next fuel order I'll just get the components and mix myself, looking at maybe a 7%syn/4%castor oil mix..

(hmm 18% oil? - http://www2.towerhobbies.com/cgi-bin/wti0001p?&I=LXNE12 but other sites have bottles at 12% , the difference is silver & green text)