oil

15W40 is a diesel-rated oil and has additives to keep the soot (carbon) in suspension.  10W30 is an automotive oil and is probably a detergent type with no soot suspension additive.  If this is a diesel, definitely use the 15W40.

Rich is correct.

Here is an example (I picked Chevron because it was convenient, and I use it. )

As you can see from the example, 15w/40 is a superior lubricant over the 10w/30, and this will be pretty much the norm across the market spectrum. The “LE” designation is intended for use in post 2007 engines.

(I was surprised to see the Delo 710 lubricant listed as this is an oil specially formulated for use in our 4,000 HP EMD 710G engines).

If that's really a 3hp gas engine, then I'd use 10W40 or straight 30wt.  I'd guess that they specc'ed 15W-40 because they don't really understand that in our US nomenclature that s a diesel-rated oil and the gas equivalent is 10W-40.  YOu don't need the soot suspension additive in a gas engine, but the detergent isn't a bad idea, so I'd use a gas engine oil.  In my climate, where it never gets really cold, I generally use straight weight oils.  Straight 30wt for all my small gas engines – until they get so sloppy they need heavier oil to keep from fouling plugs. 

You can use any of those oils and you will be fine, but I'd prefer the 20w/40. The 10w/30 may be easier to crank in cold climates.

3 HP equates to only 2.2 kW, and that's not figuring any losses.

One reason a person might want to run a 15-40  diesel engine oil in a gas engine  that is a standard ohv with standard (non roller lifters) or cam -solid lifter including hydraulic solid lifter, is reduced wear on the cam/ lifter  wear interface…

  Since the oil companies  (by order of the EPA) have been reducing the  ZDDP (Zinc Dialkyl Dithio Phosphate) -contains Zinc and  Phosphorous materials that are sacrificed -instead of the metal of the cam and lifters  due to metal to metal contact- resulting in premature wear.

All diesel rated oils have more of these sacrificial metals  which can save wear of the solid lifters and cam.. Some people buy the  ZDDP additives  and add them to for high $ 60's muscle cars. Personally I run Shell Rotella T in my 69 GTO engine and  some other older American gas engines…  my 2 cents

I run Rotella T 15w40 in everything I own and have done so since the 1970's when Cummins Engine Company had serious issues with Chevron Delo 400. I consider the extra protection the Rotella T provides worth using it over anything else on the market.