My Generac is a different animal: propane powered 13K whole house unit that's 7 years old. In my early oil changes using 5w-30 synthetic blend, I noticed more oil consumption (like 2/3 qt every 20 hours operating time). Since the 3rd OC, I've used Napa 5w-30 full synthetic with much less consumption. Whatever viscosity you choose, I say to stick with full syn.