I don't have an 850 but with the 760 and the S40 now, that light is only a reminder to replace the oil and typically comes on at 5K kms. On the 760, you turned it off by pressing a reset button on the front of the cluster and on the S40, you hold down the odometer reset button and then turn the ignition switch to on for a minute. I'd have to check the exact procedure but what I'm getting at is that there likely is a similar procedure for the 850.