n is the number of electrons transferred per one mole of the substance (just as ΔG). ΔG is most probably defined as kJ/(mol of propane), as propane has coefficient 1. The net oxidation of the carbons is -8 while in carbon dioxide it is +12, so 20 electrons are released. If ΔG was expressed through moles of oxygen, then it would have a five times smaller value, also n would have a five times smaller value, too (as 4 electrons are needed to reduce a molecule of oxygen), so there wouldn't be any difference, as it shouldn't.