Mix your 2 stroke and 4 stroke fuel (50/50) I have mixed morgan green and morgan pink, works like a charm in my OS Surpass 52 and 70. By mixing the two fuels you are getting castor lubrication (good for all those moving parts of a 4 stroke), but not that much to give you castor burnup on the valves. Allso by mixing the two fuels you keep the oil content exactly the same.

This solved my problem of carrying two types of fuel to the flying field.