If that is the case, then I think a good test would be to fill the tanks entirely with the synthetic version on a cargo plane, make a few dozen national flights

If that is the case, then I think a good test would be to fill the tanks entirely with the synthetic version on a cargo plane, make a few dozen national flights, then a few dozen international flights, log the usage per distance so that we get a table with the total cost. This test should also invoke full engine maintenance before and after so that we also get an idea of how much more or less stress and wear this puts on the engines.