Check the oil that's in the car; had mysterious oil consumption on my 560SL (148K) after the first oil change under my ownership. Found the shop had used 10W30 in a Michigan summer; took it out myself and put in 20-50 as per manual, in a high-mileage-engine formula. Now I'm making 3000 miles to the next change without dropping to the point of adding oil.
'97 SL500, 40th anniversary edition
'04 Olds Bravada (SWMBO's)
'06 Lexus ES330
'89 560SL (sold)
SL--Anything else is just a Mercedes.
(Kudos to whoever said it first)