Hi, Mark.
I've broken in around ten of these engines, both the R/C version and those on which I did my venturi conversion.
9-4 or 9-6 props seem to work fine.
I run them a bit rich for 5 to 10 minutes at a time, allowing them to cool between runs.
They will usually be ready to fly after around 4 or 5 runs.
I was originally using 10% nitro, 22% 50/50 castor/synthetic but I switched to 20% nitro, 20% synthetic with a few ounces of castor oil per gallon.
Never had a problem.
Hope this helps.
Bob Z.