I have a 98 ML320 with 32,000 Miles on it

recently when i'm on a long drive when i'm running say 50 to 60MPH at 2000 to 2500 RPM continuously for 5 to 10 mins the OIL HI indicator beeps and continually flashes until i slow down or stop and rstart the car. This doesnt happen in stop and go traffic only on a long uninterrupted drive for at least 10 to 15 mins.

I'm due for and oil change so i'll do that in a day or so but aside from the oil change can any other thing be amiss?