RE: swap prop or replace engine?
IraqVet,
Sig Champion fuels are 50/50 castor/synthetic lube at 20% total oil.
PowerMaster GMA are 50/50 castor/synthetic lube at 22% total oil.
These fuels are great for this engine.
There are others.
On a trainer, the 11x5 size would be more suitable; better breaking performance, although it will spin a bit more slowly than a 10x6 prop.