I had a '94 with the 3.2L V6 in it, and had similar issues. I could always tell when it was time for an oil change because the pressure would drop and it would start tapping. Fresh oil at the right level would bring it right back into step and no worse the wear. The drop you're seeing in typical pressure (85 -> 79) is most likely just the result of the engine being 140K old and not due to any significant issues, I would say. As long as you keep the oil level at a reasonably full state, change the filter with the oil, and don't abuse the thing, I don't think you should have any significant issues. This is assuming, of course, that you don't have any significant leaks or burning oil indicators from the truck.

FWIW, I used SeaFoam in my V6 about once a year just to clean things out. I'd put it in, run the truck about 60 miles, and then have the oil changed. Your mileage may vary.

James