User manual is silent about synthetic oil
I already have the generator so I am not buying a diesel generator. Just want to know if it's OK to use synthetic in generator engines?
Any name brand (Mobil, Castrol, Shell, Chevron, Schaeffers, Amsoil.....) Sx rated synthetic in the proper viscosity range (spec'd by generator manufacturer) will be fine in a gas gen. There are plenty of private label synthetics that are fine too, but until you are more familiar with them, don't go into just any corner store and grab whatever is labeled Snythetik.....
The one trap some people fall into is installing synthetic, then basically forgetting to change the oil, or at least running it far longer than it should be.
Modern conventional oil has come a long way in even the last 10 years, but a good synthetic will still give you better extended temperature performance. Esp. down your way, air-cooled engines deal with a lot of heat - the main factor that breaks down oil. You probably don't have my kind of cold to deal with, but I do have to remind myself that deserts see colder temps at times than I would expect - synthetic in cold extremes will be easier starting.
Initial cost shouldn't be an issue with synthetic, or OCI, as the class of gen I think you are talking about probably doesn't take much more than 2.5L of oil.
On an air-cooled engine spec'd for straight 30 or 10W30, this is one of my favourite oils:
AMSOIL Formula 4-Stroke Synthetic Small Engine Oil
If your gen will see storage time (ie. not run weekly), I'd recommend using a gasoline stabilizer additive. My current favourite for small-engine metal tanks is:
STA-BIL 36 deg Performance - Fuel Treatment For Engines
Avoid ethanol tainted fuel if possible.
Rgds, D.