I second the opinion about your valve stem seals being in bad shape.
But replacement may not be necessary. I had the same problem.
I switched to Castrol High mileage or Valvoline Max-Life, I only use 20W-50.
Before the switch I used about 1 quart of oil per 500 to 750 miles; sparkplugs needed cleaning at every oil change too (dark crust).
After 4 oil changes (at 3000 miles intervals) now the car uses 1 quart every 2500-3000 miles!!!, and the sparkplugs look clean.
This is not a snake oil comercial. I do not work for either oil company.
I am just sharing my experience with these oils.
I believe another contributor in this forum had a great experience with them.
This may not work for you, but is worth a try.
1999 Porsche 996 Carrera Convertible
1994 420E - SOLD
1986 300E - SOLD, what a car