You might want to try one of the new oils from higher mileage cars, such as Valvoline's MaxLife.
I was running about a quart every 1k and it has risen to one quart every 1,700 to 2,200 miles. A lot cheaper than even valve stem seals.
My car that I use it in has 182k miles.
'03 E320 Wagon-Sold
'95 E320 Wagon-Went to Ex
'93 190E 2.6-Wrecked
'91 300E-Went to Ex
'65 911 Coupe (#302580)