ReidMcT - Wrong! The only thing right about what you posted was "clean out the exhaust".
Sounds like the hoses were wrong; pre-mix is fine, if not better, if you don't mind mixing just leave it the way it is. If you do decide to keep running pre-mix i recommend removing the drive shaft for the oil pump; it's easy, loosen the clamp around the pump, lift pump out, remove drive shaft, reinstall pump, tighten clamp. Make sure you block off all the old opening for the oil system.
No more oil needed in the tank.
50:1 Synthetic, 40:1 conventional.
Run a BP4HS plug.