[wakespeak]

Oil gets diluted with fuel and water (from humidity). It accumulates dirt, metal, ie corrosion, rust, combustion by-products, from engine internals smaller than what gets filtered. Stretching out the low cost of fluid changes seems a bit silly if you think about the total cost of the car. Change at a mininum of once a year if you don't track the car and are low use.

Transmission and diff last over years if not tracked because there is no combustion process involved. Track use elevates the temperature much higher than street use which can break down the oil. Also there is more wear debris in these unfiltered components.