Not sure if I've misled...
Service was done just before I bought car, light reset. I was wondering if I changed the oil myself in between service intervals would it turn the service light back on. The more I think about it the dafter my question seems! It can't possibly know unless there's a sensor on the filler or drain plug, must be as said, miles or interval.
Suppose I just wanted to be sure, I hate errant lights on the instrument panel that's why the TPMS one is doing my box in at the moment. Had an Alfa 164 that reported low oil level when it wasn't, couldn't sort it, drove me bonkers.