I'd think you're best bet would be to get an engineer from Generac on the phone. The big unknown is how much you're going to use the thing. Let's say for sake of example the normal life expectancy is 200 hours but using the wrong oil reduced that expectancy by 10%. Based on the last year, it would be 18 years before you'd need a new generator. What if you have a bad couple of years? What happens if the power doesn't go out for the next 5 years?