WARNING about O-Rings: any O-ring you buy "over the counter" in a local hardware store, regardless of whether it fits or not, is in all likelyhood made of EPDM rubber. This is not compatible with gasoline, oil or most solvents, and will not last that long. That's one reason they are cheap.
Find an on-line source and get your O-Ring made of VITON. There are plenty of places that will carry them, including many hydraulic supply houses.