If you pull the cover off your genny you will see a tag with various info printed on it. There are 3 or 4 temp ranges and oil weights for each. For me, I topped the unit off when new with 10W40 (I figure the factory fill is 10W30 but I had some 10W40 at home so I used that for top off) but I will go to straight 30W when I do the first service as the average daily temperature around here would be in that category.
If you ask what brand name of oil to use you will start a war on this forum! Since the gennys are Onan brand, just buy some of that brand when you pick up a new filter. It will be blended to Onan's specs and will serve you fine. If you decide to buy another brand of oil, just remember you are dealing with an air cooled single cylinder engine.
Your RV dealership will have Onan brand or be able to tell you where you can find it. Any motorcycle shop will have oils that will be good in this application. In fact, any major brand of motor oil will do the job with no problems. The important thing is to select the proper weight from the chart on your genny for the temp range you will operate in.
2015 Jayco Precept 35UN
Towing JEEP TJ with Roadmaster plates and bar w/Blue Ox Patriot brake
an Aluma low bed trailer with HD Road King and/or Polaris RZR onboard