What brand, and weight of oil are you running? It's possible to make a big difference in oil consumption at times by simply switching brands and/or viscosity of oil. When the truck was new, it probably ran a straight-weight oil, most likely 30 weight in the summer, and possibly 10 or 20 weight in the winter, depending on the temperature. Variable-viscosity oils, like 10W-30 for instance, flow like a 10 weight when cold, and lubricate like a 30 weight when hot, but they don't increase in thickness. It's a lot easier for water-thin oil to get past piston rings and seals than a thicker oil. Just as an experiment, try running a straight 30 weight detergent oil like Shell Rotella 30HD (diesel engine oil). If that switch reduces or eliminates your oil consumption, you've found your problem. If not, you're only out of pocket the cost of a few quarts of oil!
Jerry
