Engine Warning Light !

Emergency question - after driving in city traffic for a few weeks, today my car was put on the highway. After 30 miles at speed varying 80 to 110 mph, the engine warning light came on. (It was the orange light with an engine shape with liquid wave at the bottom.) My temp. was 80C and the oil pressure was 3. I stopped, checked the oil level and looked the engine over. All was fine. However, the light went out when I was back in local traffic. Once back on the highway though, it came on again.

Is this an indicator light for something else I'm overlooking? There is no lose in performance, stalls, etc... Just the light.

Thanks.

1989 190E 2.6
103,770 Miles