My Generac 6500 has not been started in over 18 mths what's to be done

As far as I know, an engine is still an engine. Fill the oil to the proper level, put some fuel in the tank and start it up. Although, if you mean you just left it and didn't empty the fuel tank nor O_M_G SAY_IT_AIN'T_SO you didn't use Sta-bil. Then, you should be fine.

- Sorry but, I've never used nor needed nor seen any value in Sta-bil and I've let cars sit for up to 6-years WITHOUT IT...started right up. As long as you don't let the fumes out and therefore air in, the gas stays the same or real close to it. Go ahead and start-her-up. If she won't run, only then will you need to dump the old fuel. Never pour new into old, but do try a bottle of "Octane Booster".