The oil light comes on at one quart low. Mine uses about one quart on 1500 miles or so -- my Mother drives it, so I'm not too sure, but it's not much.
All the rubber vac lines are probably rock hard or melting with age, replace them all. Will make the tranny shift softer, too.
High temp at idle in traffice (the line above 80C or so) that drops quickly makes me think of bad visco clutch. Mine is, and will be replaced this week when I do a new belt and fix the AC (hopefully the expansion valve is the only leak....). You can test the visco clutch by listening when the temp is up -- you should hear it blowing, it makes more noise than when it's not locked up. Should also move much more air. If you then shut the engine off while watching the fan, it must stop within one revolution. If not, the clutch is bad and needs to be replaced. Takes two "special" tools -- an 8mm allen on a flat bar (or in a flattened pipe like we made) and a 4mm allen or 1/8" wire to lock the clutch to the carrier so you can unscrew to hex socket bolt.
1972 220D ?? miles
1988 300E 200,012
1987 300D Turbo killed 9/25/07, 275,000 miles
1985 Volvo 740 GLE Turobodiesel 218,000
1972 280 SE 4.5 165, 000 - It runs!