Do you use an air filter?

I've noticed that car fuels tend to be lighter on oil content than a lot of airplane fuels. You might try using airplane fuel of modest nitro content (5-10%) with some castor oil in it. I've had airplane engines run for many hundreds of hours with low nitro fuels that had castor oil in them. The one's that have been whipped hard with 40% nitro in competition tended to wear out and break faster than the more conservatively run ones.

Any engine take a while to break in properly too. Even the supposedly "quick" breaking in ABC's can take 45 minutes or more of gentle treatment before they're really ready to flog hard. Rush the process and longevity and performance ultimately suffer.