I agree that the higher RPM's from the highway drive is making the oil consumption worse. My 90 300SE uses about a quart of reg. oil every 1000 mile of driving 75-80 MPH tach at 4,000. I use the new 0-40 and 15-50 Mobil 1 syn. and oil consumption has gone down to about a quart every 1400 miles. The valve seals were replaced at about 110K and oil consumption with the new seals was about 1 qrt reg oil every 2000 miles. I didn't think that was right so the dealer did a leak down test and found 2 cyl. to have a higher leak rate then normal but not bad enough to worry.
Why did you have your head rebuilt?
1969 RS/SS Camaro Conv. L-89 Alum. Head 396 375HP 4-spd
1987 GNX #419, 2002 RX300
FS: 1934 Ford Street Rod, Blown 383 $48,500 Firm