That is not completely correct, the engine manufacturer does get to specify -grades- of fuel, and if you read the operators manual of a K21/K25 dual fuel MCFA product, you will see that as far as grades of gasoline, they specify only a high test 92 octane for running on gasoline and an HD5 for LPG fuel, and if you run regular 86 octane, you will find that it does not always run smoothly (usually the performance complaint is related to idle).Since these were mainly sold to rental houses, I have always wondered how you could possibly get a short term rental user to put only hightest in the truck before returning it.
You also break federal law (as if anyone would notice or enforce it), and end the emissions controls parts somewhat longer warranty by using a fuel of less quality than specified.
As far as I know, MCFA did not make any gasoline -only- tier 2 trucks, nor have I seen anyone else who has a tier 2 gasoline only truck (That does not mean they are not out there, just that I have not run across any).