If you have no idea what kind of oil it had, you may want to just have it serviced and get the oil changed. You really don't know if it's "Dino" or Synthetic, and you should keep the same oil in it.
I don't know if it will do any damage, if you were to add some 10W-30 to it, but it might be best to do an oil and filters change....for a bit of piece of mind.
Check/replace the fuel and air filters will be a step in the right direction. How many hours on the unit?
Bill & Claudia / DD Jenn / DS Chris / GS MJ Dogs: Sophie, Abby, Brandy, Kahlie, Annie, Maggie, Tugger & Beau RIP: Cookie, Foxy & Gidget @ Rainbow Bridge.
2000 Winnebago "Minnie" 31C, Ford V-10
Purchased April 2008 FMCA# F407293 The Pets