So my "OIL SERVICE INDICATOR" has just come on but I can't get to a garage for 6 days.[&:]In that time I will drive about 100 miles (some around town,some motorway) Am I safe or should I not use the car ? Would "topping up" with clean oil be of any use ?

 

You would be safe, it's just telling you it time for the service, it
doesn't have to be done just as the indicator goes off, but soon.