Your owners manual explains that in some detail.... it's nothing to worry about. It runs on a clock based on miles driven. Turns on every 10,000 or so to tell you to go to Volvo and pay $50 for an oil change. You can reset it in most 900-series cars this way:
remove little black rubber plug in lower left of your instrument panel. Don't let it fall inside, pull it out to you with your fingernail or a small screwdriver blade or similar. There's a button behind it. That's the reset button. Use a pen or something small and pointy and push it in. That's it, resets it for another year or so.
Now if it said "Check Engine"... that's somethign to worry about. The Service light is supposed to stay on for 4 minutes or something after each time you start the car.
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Rob Bareiss, New London CT ::: 86 244DL, 87 244DL, 88 744GLE, 91 244: 808K total