Just to follow up -- I had the exact same experience as you. A gas attendant filled up my X5 (in NJ, full service is required by law) and 24 hours later, the SES light came on. I bet that you just have a harmless gas cap/oxygen in the system issue. If your car is otherwise running fine and you have enough patience to wait it out, it will probably go away eventually. You will feel like an idiot (like me) if you take it to the dealership and get charged an arm and a leg for that diagnosis.

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2000 Oxford Green X5 4.4
2002 Titanium Silver 325i Sport