Oil level at midway between the marks can sometimes cause the FSS to signal low oil on the highway.
At 60,000 miles I'd get the engine checked for leaks, as you shouldn't be using that much oil already, even though I'd not consider a quart in 2000 excessive yet. It does sounds as though this just started, though -- that indicates a leak to me.
One thing to look at is the oil filler gasket -- a leak here only shows up at highway speeds, and the oil won't be dripping anymore by the time you park the car.
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!