We bought our 850 NA about three years ago with only 62,000 miles on it. Although there was no service history with the car, indications were that it had been at least adequately maintained. But it always had a bit of consumption, about a quart every 1000 miles. I don't like that, but decided to live with it. It is NOT leaking, and the car does puff blue at start-up

Since purchase I've changed the oil at no more than 5000 miles, always using either Castrol or Valvolene, an always a Mann filter. I removed the flame trap, but the rest of the oil trap and tubing is original.

The car is my wife's and is used mostly about town, though it gets frequent trips long enough to get the oil hot, and occasional 100+ mile road trips..nothing major.

Oil consumption has not changed, until now. We took the car on a 450 mile trip, mostly interstate, and in that time we managed to burn 1 ½ quarts. It’s only been driven about 50 miles since, so I don’t know if the consumption is permanently high, or if it is related to highway travel.

Do any of these clues offer any suggestions of where to start looking? Do the 850’s have rubber valve seals, or just guides (thinking of start up puff of blue) or could it be the oil trap? Or is it really likely to be a deeper issue like rings or valve guides?

There's a vacuum hose connection on the flame trap that tends to get plugged up w/ hard carbon-like material. It may need to be drilled out. Loosen the oil filler cap while idleing. If it starts bouncing around it's an indication of high internal pressure. Maybe do a compression check?

A leak-down check is on the agenda for maybe this weekend if I have time, or more likely the following weekend.

Would that vacuum line by itself create high crankcase pressure and thus excessive oil burning? In fact would any cause of high CC pressure cause that, or just leaks?

A would think that any blockages in the PC system would tend to affect internal CC pressure and agrivate oil leakage around any worn components. From the description of your start-up "blue puffs" it appears you may have some worn valve guides. Maybe checked the intake system with a vacuum gauge would show something?