Ring wear should show in a compression test right? Does anyone know what the range should be for this test - and any tips for getting at the injectors on this engine?

The TD is notorious for that. Like melmac said... the injection pump is supplying more fuel than is necessary.

Has it gotten worse, or is it an "always this bad" kind of thing?

The reason it smokes less after highway travel is because a lot of the residual junk that blows out along the way, has been dumped from the exhaust system. If you are good at watching your mirrors, watch the rearview as much as possible when you're on a highway trip. Periodically there will be a big, quick explosion of black smoke. I always attribute that to debris in the exhaust loosening.

If you're seeing gray smoke, there is either a timing issue (don't adjust that yourself!) or an oil burning issue.

The diesel turbos usually lasted for a long time.
--
1992 940 wagon, 72k

Black smoke? Blue-gray smoke? I grew up in farm country and a tractor putting out lots of black smoke meant that the manufacturer or tractor shop had turned up the pump or put in higher capacity injectors to increase the horsepower at the expense of efficiency. The black smoke was incompletely burned fuel.

Later as a general aviation mechanic I saw the smokescreen thrown by turbos with ruined bearings, which relied on air pressure to keep a good seal. Wear out the bearing or shaft and the oil pressure in the center between the hot and cold section bearings has room to squirt through to the exhaust. Raw oil in the hot exhaust is pretty dramatic, throwing a blue-gray fog. Also goes through oil really fast.

Moderate blue-gray exhaust with managable oil consumption would probably be indicative of ring wear. I have no idea how much is too much for a diesel. With gas its until you can't stand it or a worn out piston breaks or you don't catch a really low oil level in time and fry it.

Mel