These cars are smart? Check engine light reset itself?

About a week or so at 3,000 miles the check engine light came on. I tightened up what I thought was a loose gas cap and called the dealer.

He tolded me not worry that it would reset itself and all was well. After another 100 miles or so, it did not go out so I called and scheduled an appointment. Last night when I started the car, the light went out. About 300 miles and 2 more trips to the gas station.

I am going to cancel my service appointment. Has anyone else experienced this?

Additionally, has anyone added oil to their engine between service intervals. If so, where did you buy the oil?

Thanks. CM