2002 Pontiac Sunfire Check Oil Light

The oil light comes on in my car after it has been driven for 15+ minutes and is idled, but the light goes off after its driving again. The first few times that it happened, it took a much longer trip to make the light come on, but now the time in decreasing. The oil level is fine, the oil was just changed and the mechanic put an additive in it. I have taken it into the shop twice now with no results. They just try something else, charge 50 bucks and tell me to cross my fingers! They said that it shouldn't be serious as long as the light doesn't stay on all the time. They said that the pressure sensor isn't overly sensitive, because they checked it. Please help! Should I be concerned? Should I take it for another opinion somewhere else?