If you spend a lot of time cruising down highways, especially in the winter, a combustion engine is inevitably almost always in service, unless you’re decelerating, driving downhill or stopping. As a combustion engine is doing most of the grunt work, you’ll need to perform regularly scheduled maintenance, including engine oil and filter replacement. In the case of the Toyota Prius, the manufacturer recommends performing this service at 8,000km intervals.
On the flip side, if most of your driving is of the city variety, chances are the electric motor carries the brunt of the load. City driving also implies a lot of braking that, through the regenerative braking system of your hybrid vehicle, keeps your battery charged, which in turn allows you to use the electric motor even longer. In this scenario, the combustion engine is rarely solicited, will never overheat, and therefore extend service intervals such as oil and filter changes into the 15,000km or 12-month threshold.