So after sitting at a stoplight, or other idling situations I get a bluish-white smoke cloud. I assume oil is being burnt, but here's where it gets a little weird:

I changed oil, put in 6 quarts and drove ~750 miles. It was on/below the line for 1.5L low.

Filled with another quart, and it really hasn't changed in the last 400. I assume most of the "missing oil" is what goes into the filter. (Though I think engine is 6.1 and filter (MANN) is .5, so where is the other quart? The dipstick still reads approximately .5L low, but I'm not concerned about that. I've put 7 quarts in, total capacity is 6.6 and I'm 1/2 quart low.)

PCV system doesn't appear to be clogged, no puffing out of the dipstick. Last cleaned it 20k ago.

That leads me to believe that the valve stem seals are going. Any projections on timeline before it isn't worth it to drive the car? She's got 243k on it, and I'd like to get at least 250 before I retire it. It has no A/C, some body damage, and probably needs some brake work shortly. I don't think I'm going to replace the head (I figure a rebuilt head has everything I'd need, and then some, to keep it running a long time, and I don't believe it is prudent to try to replace without taking the head off.

So, I guess I should get to a point about now: Any idea if the smoking is just an old engine being old or sign of a huge issue just down the road? If the latter, what is the best way to proceed without breaking the bank? I'm debating on a leak-down test, but I don't think the rings are to blame.
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If you're not driving it "like its stolen," are you really driving?