One thing Dar and I agree on is to use a fuel with 25% oil for running in. I prefer all castor but so long as there's a good proportion of castor then that's fine too. I'm fussy about running in because I want my engines to have the longest life possible, they're not throw-away items to me. Nitro when running in is a waste of money but it doesn't take all that much fuel to run in an engine so I'd suggest getting a fuel with the lowest nitro content you can find and fly out whatever remains. If you make your own fuel like I do then just knock up a quart or so of zero nitro and 25% oil.