ALVO360;c-138520 wroteWhat breaks oil down anyway?
The main thing that breaks oil down is oxidation. When the oil is hot, oxidation occurs faster. If you heat oil up too much (quite a long way above engine OT) it will experience thermal failure and begin to break down. Even in a healthy engine, this can occur in small areas, like between a piston and cylinder wall, where you get small pockets of high pressure and heat.
@Vee_Que works in the industry and is of course right about what manufacturers recommend, but they have to consider lots of external factors, like owner habits and maintenance costs, which may affect the perceived cost and reputation of their cars.
I don't think changing the oil at 5,000km intervals but changing the filter at 10,000km intervals creates a false economy. The filter is still doing its job that entire time, which is to scrub large contaminant particles from the oil.
