Pretty much anything you need to do to the fork will involve taking it completely apart and rebuilding seals. That is when you will replace the oil. It shouldn't need all that. There are some forks that need a little oil on the dust seals occasionally, you'll have to check if that applies to yours. I don't know if any documentation is available on older Trek forks, you might check with a bike shop that is a Trek dealer, or maybe it can be found online. If it holds air, it's probably alright, though I've seen some bikes have an air spring in perfect condition but have problems with the damper. If you can get the parts and are willing to make a mess, rebuilding it would be a good learning experience.