Just out of curiosity, how many of the categories in the study which I assume are based on gasoline fuel, would have had different results, better or worse, when using propane or natural gas fuel?
I also have some concerns with the operating cost chart. The oil manufacturer recommended change intervals do not agree with either Generac's or auto manufacturers owners manual recommendations. If a warranty claim were to come up with a major engine problem, I am pretty sure that the equipment manufacturer would not honor a warranty if the problem was traced to not following the manual's instructions regarding frequency of oil changes, no matter what the oil manufacturer's recommendations are. Showing costs based on the oil manufacturers recommended intervals, versus the engine manufacturer's specified intervals is misleading, in my opinion.
Propane and Natural Gas burn much cleaner then gasoline. I think it is safe to assume that propane and natural gas could only influence the test results in a positive way.
Keep in mind, some of the tests are not run in an engine, under real world conditions. Therefore, fuel would not influence the test results.
For instance, in the NOACK test, a candidate oil is exposed to heat and circulating air. Following 60 minutes, the remaining oil volume is weighed and compared to the original weight, with the difference reported as the percentage of weight lost. Results must be limited to 15 percent or less to meet the API SN and ILSAC GF-5 specifications.