I change my diesel truck's oil every 10,000 km, that's 6,200 miles and is an easy round number to keep track of. Or every 6 months if I don't have much usage. Fuel filters every second oil change or 20K km. More expensive than gas? I would say so. I buy 15 litres of 5W-40 synthetic at around $150 after tax (Canada, eh), oil filter is $35 at the dealer and fuel filters are around $100 for the pair of them. My car takes 5 litres of oil for $35, and the filter is $15. Cost of two services on the truck is $470, on the car is $100. But - the car gets a service every 15K km. That makes the truck basic maintenance roughly 6 times as much as the car. I do the work myself and I can dispose of the oil for free.
That's the factory interval for both vehicles, btw.
Brian