When the oil cap seal gets hard and no longer seals, you can lose a lot of oil. I was losing a fair quantity on the 4.5 until I replace the seal. FastLane and PartsShop sell the seals for about $.30, much cheaper than a new cap. Blowby will greatly increase the rate of loss. How many miles on your car?
'73 280SEL 4.5 (9/72)- RIP
Only 8,173 units built from 5/71 thru 11/72
'02 CLK320 Cabriolet - wifey's mid-life crisis
2012 VW Jetta Sportwagon TDI...at least its a diesel
Non illegitemae carborundum.