My wife's 2009 SLK350 is experiencing unusual oil consumption. I have researched this site and only found issues relating to the 2005-06 models. There are no leaks that I can tell or see (no drops on the garage floor). My wife drives approximately 100 miles a day and is using about 1 quart of oil (Mobil-1 0 - 40W) every 2 weeks. Does anyone know of a known problem with this model or should we be using a different weight of oil? She has approximately 160K miles on the car. I'm worried that the piston rings may need to be replaced, which could mean big dollars to have this done. Not sure how much, but I'm sure it won't be cheap!
This is definitely high, but does not have to be absolutely excessive ( 1000 miles = 1600 km / 1 quart = 1 liter).
Either, the oil is leaking into the combustion chambers, or out of the car. These amounts will be very visble, especially with the miles put on the car, so if there is a outgoing leak, yo'll see it (wet underbody panels).
If the oil is sucked in via valve stems (and bad valve seals, before that) the valves will accumulate soot and slowly coke up, ultimately giving bad performance and risk of burnt valves. Sometimes, this can be observed by a puff of blue smoke when starting, and perhaps not much thereafter.
The other way is thru the cylinder walls. Sometimes, cylinder walls are "polished" (way more complex that the word "polishing" implies) sometimes piston compression rings and oil scraping rings are stuck in deposits. Either (or both) make cylinders prone to oil leakage and hence, oil consumption. You will not catch this with a simple compression test - sometimes a proper leakdown test (compression testing for a longer period, measuring leakdown rate) can help pinpionting this. (However, it will not pinpoint if cylinder walls or piston rings are the problem.
Now to a suggestion: It is possible that some "italian tuning" could help. The reason is that ordinary, sensible driving will not load the engine enough to get really warm, and driving at high revolutions (at reasonable speed) may free somewhat stuck rings. So, take the car for a good spin and make sure it is redlined (or nearly so) and driven at speed up hills (to get hot). I will not promise anything, but it's nearly free (and somewhat fun) to do this and it may help, seriously.
If the oil is leaking into the combustion chamber either way, it will be burnt. If your car burns these amounts, it is not absolutely certain that it shows ("blue smoke") but it is not good for the cats and as a (bad) fuel , Mobil1 is rather expensive...