Ed, that's not what I was not saying.
I was saying that when EPA set the emission standards, the standards must be met by the engine manufacturer in order for the engine to be EPA certified.
The engine must be able to meet the standards whether the truck is configured to burn gasoline, or propane, or configured as a dual fuel unit that can switch between the two fuels at will.
And yes, the OEM can specify that premium gasoline or HD-5 propane be the only approved fuels.
EPA only requires that unleaded gasoline be used in EPA certified engines, EPA is not concerned with octane rating.
It is a violation of Federal law to burn leaded gasoline in an engine that is EPA certified.
Komatsu calls for gasoline of 87 octane minimum and 107 minimum octane propane with K-21/25.