Thousands of ancient NHE Onan generators operated just fine to 8,500 feet and even up to 11,000 feet with NO altitude adjustment screw. If I were you I would certainly adjust to 5,000 feet even when at 10,000 feet altitude and TRY IT OUT before I started to worry about the sticker. Only electronic fuel injection has made any significant difference in how a genset behaves at altitude. 11,000 feet? Yes. Saddlebag lake RV campground which is actually at 11,200 feet altitude off Highway 120 atop Tioga Pass California. I serviced RV's and the facility generators up there for years. Adjusting choke settings and cautioning owners about a severely limited maximum rated output turned out to be the more significant points. What DID make a huge difference was replacing a WITTE diesel genset with a turbocharged Perkins.
