Can be other things but the glow plugs may be the old style and aged. New pencil plugs with the correct diameter mounting thread are still available.

If you have the old style series glow plugs you will see curly wires between some of them. These tend to be resistant dropping wires. Too much heat energy then does not get to the pre chambers. It is lost on the jumper wires.

Installing the new pencil type plugs since they are no longer in series requires some rewiring of their hookup. Most people that have done this conversion have thought it really improved things.

I was also wondering if the car was starting with no actual glow plug cycle. Usually your engine would not in my opinion at the temperatures you quote unless in very good condition. It is possible though as well.

A preliminary test just in case you have one glow plug shorted out. Get a glow cycle going and measure for each glow plug heating up with hopefully a laser guided type temperature reader.

A base oil actual consumption rate of 300 miles per quart puts this engine somewhat into the fairly worn category condition. Have a look for oil leaks or the vent line from the vacuum pump having oil in it. Both these problems can drop the base oil level quicker and make the engine seem like a heavy oil user when it really is not.