That would be in the Ford E-Series owner's manual in the Thor bag. At least for the V-10. Don't remember if there was a generator manual in the bag (if there is the genny's old spec should be in it).
The data is in the Ford owners manual near the back page. For the Triton V-10 2 valve engines Ford usually specifies Motorcraft semi synthetic 5W-20. For my coach on the F-53 chassis with the Triton 3 valve engine, Ford specifies 5W-30 Motorcraft semi synthetic. I use Mobil 1 full synthetic in the 5W-30 weight.
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Jim & Roy Davis
2016 Hurricane 31S
1961 Rampside in tow