Extra oil won't hurt during break-in, I use 14-18% oil to break in most of my car engines. Having extra oil allows a leaner needle setting which gives more rpm which gives more heat which helps the engine break in. This may make my method sound crude or wrong, but it works for me very well. Even in Traxxas 3.3's. I do change over to 20-30% nitro and 8-10% oil after a half gallon or so.
So I don't think fuel is the problem per sé but fuel mixture could be suspect. Too rich on the Low could load it up and cause a hydraulic lock. Check this first, pull the plug. If the piston won't come loose after it's cooled and sat for a few hours even with the plug out, I'd hit it with a hair dryer and get the engine good and warm (hotter is better IMO) and try moving the flywheel again. If nothing still, I'd proceed to pulling the engine and inspecting. I much prefer the more simpler approach at first. Sometimes it can save a lot of time.