Weve got an ultra, but same chassis And engine as yours. The 8 liter is a good engine, good torque. We run synthetic oil changed yearly. Have 50k on it already, bought new.
The only big thing we ran into is there was not a tranny cooler originaly installed, odd but chevy listed it as option on the gas rv chassis (04) and standard on almost All others. Just check To see if you have one.
After monitoring tranny temps 2 yrs ago, put an oem cooler in and only got it to 190 climbing the steep grades on the Ice Fields pkway by Calgary last summer. Ran 160s normal non-climbing with the cooler. Before we ran low 200s and into 250-60s in the mountains.
Mileage for us runs 6-7mpg averaging 63mph Over the life.
2006 Gulfstream Ultra 6331
33' Super C
8.1L V8 Gas