What is the usual service interval for my car.

Has the variable service option, but is now displaying the service due countdown.

Just past 8k miles.

18 to 20k as I understand it with mine.

It's worked out by several factors. Mileage covered, oil condition as detected by a sensor, number of engine starts, and days elapsed since last service. I had variable service on my 05 plate Polo, did very low miles, but service indicator would come on after 16 months, even if I only covered 3000 miles in that time.