Oil consumption, Valve stems?

I have a 1990 Chev K1500 4x4 Scotty since it was new, with a 350 in it at 138,000 miles. It goes through a quart of oil every 200 miles (that's a LOT!). It's been creeping up to that amount for a few years now. No oil leaks, no blue smoke except occasionally on engine start. It runs very well otherwise. No plug fouling or missing. No concerns about the replaced spark plugs from the mechanic after a recent tuneup. I figure it is either the valve oil seals, valve guides or both.
Has anyone else had experience with this problem? Did you find a solution for it? Does anyone have enough experience with working on a 350 to suggest if the problem is usually the guides, valve stem seals or both? I have heard that there was a Tecnical Service Bulletin regarding premature valve stem seal failure on the early 90's 350. I highly suspect it is the valve stem seals. Is it practical to R and R the heads on an engine with 138K on it or is that asking for new problems?
Any comments or suggestions much appreciated. Mike H. Seattle.

Just out of curosity, what kind of oil are you using? had similar consumtion ( 92 5.0L 184kmi) about a quart every 500mi and the plugs were ashed up bad on #1 and #7 with smoke on start up. I figured it was the valve seals, but out on a whim ( and pure lazyness) i thought i'd give 5w-30 valvoline maxlife synthetic a try. 700mi later only used .25qt and no more smoke. If you dont want to try it or it doesn't work, there is plenty of info on the forum about changing the stem seals with the head on the engine. This is a very common issue with these engines.

I haven't tried any synthetic oil. Usually use a 10-40 grade. It is a little better with the 10-40 or a straight 30 wt but not much. When you are going through that much oil ya watch for cases of decent oil with a good rebate. You don't sacrifice the expensive stuff. Looking around I do see a lot of posts regarding the valve stem seals.
I understand that the "high mileage" synthetic oil is designed to help soften older hardened stem seals, much like many additives that are out there. I would put more faith in the synthetic oil itself rather than the additives. Seems it did work for you! Thanks for the idea.