Sounds like an oil cooler line leaking. best not to drive it until you replace the line, you will loose the engine oil pretty fast it it lets go. 1/2 litre on the ground after stopped for a few minutes is not a good sign.
There are a couple camps on this issue. some say to replace with OEM hoses, and others remove the lines and have a hydraulic shop crimp on new hoses to the pipe and the end that attaches to the oil cooler.
You will still have to remove the line from the engine to do the job.
You didn`t mention which model car this is, Iam assuming a W123? the hoses are around $75 each new, and are a DIY job, just a PITA to do. then after you are done, you can say it is a piece of cake to do.
If it is leaking as much as you said, do not drive the car. and if you do/did check the oil level.
There are plenty of oil cooler lines threads posted over the years.
Here is a link to hours of reading.
http://www.peachparts.com/shopforum/diesel-discussion/211352-engine-oil-cooler-hose-line-issue-link-thread.html#post1738108
Charlie