Well, you can switch back-and-forth between conventional (factory-fill) and synthetic. They are fully compatible. I have contacted Pennzoil, Castrol, Valvoline and Mobil 1, via email, about their compatibilities. I don't know why you are not getting better MPGs with synthetic oil. I did get about 2 MPG better, after switching to synthetic. The main reason why I switched to synthetic is that I wanted to my engine to last longer. Conventional oil will suffice, however. My recommendation, stick with SYNTHETIC! My xB2's engine runs noticeably smoother.