I always ran my OS91's on 10% nitro. 15% is a bit high. If you already have the fuel and want to try it I'd recommend adding another head shim.

What % oil is in the heli mix? You need at least 20%, and it should be a synthetic/castor blend.

Your best bet is to order some Wildcat Jet-A DF fuel in either 5 or 10% nitro. It has about 23% oil.

Joe