It seems the Toyota Prius runs best on Synthetic fluids and gases. With Mobil One 0-20w oil I have gained 4 MPG, and the transaxile runs on synthetic trans fluid. And last but not least I was told to try R134A+ synthetic Freon, which I have found to increase the cooling efficency at start up here in Southern Florida. Not to mention the pro green effects of man made oils and gases. So synthisize away, for further fuel ecomony.