Same here, both of ours are air cooled. We usually pick up our parts at Cummins, a little more expensive, but we know that we're getting quality parts designed for the units. Oil filters usually run $8-9, fuel filters about $10, air filters a little more. Oil can sometimes be picked up at the auto parts store on sale, we're were told by the guys at Cummins that Valvoline is the same oil that they use. There's nothing difficult about changing any of these. If you need to, order a service manual from Cummins, I think they're about $18.
We change the oil and filter on the work truck monthly, tune-ups are about every 3-4 months, and once a year, take it in for a major service and let them look it over real well. This usually runs between $300-400. But this unit gets worked every day and if it goes down, we're out of business.