Bad rings, Bad valve stem seals, Issues with the PCV system.
23,000 miles suggests it doesn't get driven much so likely the rings are
Run a compression test, both dry and wet and see what you have. Wouldn't
hurt to look inside with a bore scope and make sure you don't have a
broken ring or similar.
Then I would probably dump some seafoam or techron in the tank, do a top
cylinder cleaning, change the oil and take it for a drive. Get it warm
and WORK it some, like uphill under load. That might get the rings freed up.
The other option would be to open it up and re-ring it.
It's possible nothing is wrong with it. A quart every 1000 is
acceptable. Take a look at the plugs. If they all look the same and
are "clean" I wouldn't worry about it. OTOH it's possible you might
have one or two bad cylinders, not bad enough to make it run poorly
but with bad rings or valve guides causing oil consumption on those
one or two cylinders. That should show up on the plugs. I had a
truck I bought with 24,000 miles on it and the GM350 engine in it. It
used a quart every 500 miles, no leaks, no smoke. It never fouled
plugs and at 120,000 when I sold it it still burned a quart every 500.
Some engines just do.
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