When is an engine "done", turkey tester or not? This may be non-scientific, but you can just sort of tell. Once you fly the engine you get to know it. It flies just fine when new, usually on factory settings, then one flight it starts to "burble" just a little in the midrange. "Lean me" it's saying. From that point forward you have to lean the low end sequentially after every few flights - not much, maybe just a 1/8 turn or so. When you get to the point when it holds the setting for multiple flights and no more black goo is emmitted from the exhaust, then the engine is "done". Then, if you want, you can switch to synthetic oil. However, remember that modern two stroke non-synthetic air-coooled engine oils are very good and using them indefinitely will not endanger your investment at all.