Sounds to me like you are not measuring your oil carefully enough.

Try doing it somewhat consistently as to time (after shutdown) and place so you have an accurate idea of how much oil has gone missing.

Also 8 quarts for a change sounds like too much. At 7.5 qts, I am checking the dipstick to make sure I do not add too much oil.

FWIW, on my 2000 C280, 100k, I have a strong oil smell, particularly in an enclosed garage. On inspection, I believe it to be leaking rear main seal with oil splashing on the exhaust.