Basque I,
I may not be able to diagnose your problem, but I might be able to help you better describe it for those who can. "Backfire" means different things to different people, so tell us this.
1. Is it popping from the exhaust, especially during deceleration?
2. Is it misfiring (hesitation like one cylinder didn't fire) during cruise, acceleration or deceleration?
3. Have you changed anything else recently, other than the
oil?
4. Does the symptom only occur during certain circumstances? If so, what are they?
5. Have you checked the timing recently? If so, what is it?