this is the first reply so far! there will be more

   Chet

Take it in for service.

It's a reminder light, will extinguish when the car is serviced and
interval
light is reset.

I believe you can do it yourself too, if you do your own oil changes...
don't know the procedue on an S40, I believe it involves putting the key to
position II and holding something down... On our 960 it's much easier, as
there is a reset button.