There was a great comprehensive study done by some dude on the Intertron regarding oil filters. He bought Fram, K&N, Canadian Tire, Napa, Lordco, Autosense, Mopar, Motorcraft, AC Delco, Pennzoil, whatever, and ripped them all apart.
Bottom line: The filters from the auto makers (Mopar, Motorcaft, Delco, etc) were the best.
Fuel additives: Injector cleaner is cool, octane booster not so… there are only certain rare situations (like trying to get your POS '82 Toyota to pass AirCare) where some more extreme additives are useful.
Engine oil additives come in a variety of flavours: Engine cleaners are pretty harsh stuff (most of them are mostly kerosene, I think) that act as a solvent, scouring your cylinder walls and valves and shit. Those should only be used like an oven cleaner: put 'em in, let them do their thing, then immediately change the oil and filter.
Oil thickeners are kinda sketchy. The idea is that if you’re leaking/burning oil, this can help prevent consumption. But the raison d’etre of your oil is to flow and lubricate, and thickening it does reduce its fluidity.
Anyhow, like Geoff said, the best thing to do is get your oil changed frequently. And if your mechanic says “hey, your left handed widget o-ring is leaking” or “you’re puffing a little blue smoke, yo” and gives you a quote, maybe have a look yourself, and check for drips on the driveway and stuff, check your oil level periodically, and think about how severe it really is. As part of my job, I inform people about their oil leaks and things like that, and I really wish I could tell them, “Well, it’s going to be $200 to take that apart and change the gasket or seal it or whatever. You’re only losing a litre a month… $200 will buy you a lot of oil.”
Also like Geoff said, modern cars really like to have their fuel filter changed every few years. Unfortunately, on a lot of cars the filter is mounted right to the fuel pump, and requires taking the gas tank out to change. If it’s an inline filter mounted somewhere along the frame, though, even your Mr. Lube or Crappy Tire or whatever will have no problem changing it. Especially in coastal areas, and especially the farther you get from major distribution centers or refineries, you can be getting tainted gasoline, or fuel with too much water in it, even from the big brand name gas stations.
Anyhow, Bigthumb, do you have any particularly interest, or just curiousity? Is your ride giving you some grief?