Don't specifically remember the break in on either of my two YS .45's, but I always used 15% nitro castor/synthetic blend fuel.
It's good to have at least some castor in the fuel since you can't use petroleum-based after run oils (because of the silicone regulator parts) and the castor gives some anti-rust protection for the bearings.
I used an APC 11X10 pattern prop on mine in a Boxer .45 pattern ship. Something like an 11X7 would be good for breakin, I think.