generator

15w/40 and should be 3 qts with oil filter change. This should be ON the Onan by the Start switch, including Onan Part numbers for Air/Oil/fuel filter. Newer Onans do not have these labels. Doug

Correct viscosity may vary according to ambient temps you plan to use the generator in. 15W40 is generally good, but if you're doing winter camping in cold climates, you might want something else.

15W40 is good anywhere. We've used it from Az @ 120 to -50 in Mn.

What about Rotella there is 15-40 white bottle then 15-40 gray bottle which is synthetic blend, then the blue bottle is 5-40 pure synthetic then if you add Lucas which I have used in diesels since the 1990's it is used one quart per every five quarts and they sell it in a smaller bottle for Harleys which would be the size used in a 3 quart system I know it is another wrinkle a friend owns a trucking company and has trucks with millions of miles and he has used lucas in that mixture of 1 quart for every 5 of oil . I told him I just wanted to get it right and it is a used motorhome his r4ply was more reason to use lucas oil additive he told me he also has a bluebird and it is nice 250k on it his kids run the business mostly, and this is how he thinks the generator and motor will last longer especially in hot weather and he uses the Rotella 15-40 synthetic blend. Is the more you learn really the more you know