For Ford V10 RV engines, Wix 51348 (also sold as NAPA Gold 1348) offers the best balance of quality and value. Premium options like Mobil 1 M1-210A provide extended service intervals for RVers who drive long distances.
The Onan 5500 can be tuned up with quality aftermarket parts when OEM parts are unavailable. Use NGK 3432 spark plugs, Wix 57358 oil filter (not 57398), and zinc-containing oils like Rotella T6 for optimal performance.
For your E450 V10 with 8-16 miles monthly on dusty dirt roads, change the air filter every 12,000-15,000 miles instead of the manual's 30k recommendation. Check and clean it every 3,000 miles.
Switching from Motorcraft synthetic blend to Costco full synthetic is an upgrade, not a downgrade. Full synthetic provides better protection for RV engines. Use 5W-20 viscosity and you are fine. Kirkland oil meets all Ford specifications.
4x4 RV transmission filter service same as 2WD. Also service transfer case and front differential with correct fluids per manufacturer spec.