Oil consumption in older engines can happen for any number of reasons, including worn piston rings, oil pan seals, etc. One thing you don't mention is the interval at which you have to add oil. Is it every hundred miles or every thousand miles? The latter in an older car quite common. Both of my cars have a little more than 130,000 miles on them and I can count on having to add a quart every 1,000-1,500 miles; in fact, I even carry a quart in the trunk at all times in case I need to add it while I'm out. My driveway slants downhill enough to throw the dipstick readings off, so I make a point of checking it at work when I'm parked on level ground, and I then have oil on hand if it's critically low.
