Onan doesn't make oil - they just make a big profit re-selling somebody's oil at a huge mark-up. Ditto for Onan filters. You don't need Onan brand fluids or filters.
Just buy the grade of oil recommended in the genset owner manual, which will almost surely be a diesel grade oil (API Service Class CH4) and a viscosity suitable for the temperature range (usually 15W40 but see your Onan manual for the temperature vs viscosity ranges). Any brand that has the API class and viscosity will be fine,
Former owner of 2004 American Tradition and several other RVs
Home is in the Ocala Nat'l Forest near Ocala, FL
Summers in Black Mountain, NC