With a this kind of mileage, I'd start by checking several things.
The PCV valve is among them. If the PCV valve is bad, the oil consumption
might also go up.
There is a number of very simple things you could check or, at least be
aware of when was the last time they were inspected /replaced:
- air filter
- fuel filter
- spark plugs
- ignition cables
- distributor and rotor (unless it has a coil)

All those things if not right would cause your mileage to drop. During sumer
it is easy to do yourself but with -30 C we had last night, you need a
garage.
With time, lot of people do oil changes only and neglect to check this
basic/do-it-yourself things.