I have noticed that my 500E is losing a dark fluid which I can't identify. After being parked for two days, my car will leave a stain about the size of a quarter. It looks like oil except that it doesn't feel greasy at all. It doesn't smell either. Could motor oil dry so quickly? I can't recall ever finding a "fresh" stain.

I have to drive my car sometimes, and so I always religiously check oil and ATF levels, all fine everytime.
P/S pump has just been resealed, P/S fluid level fine.
Coolant level is ok, my car runs like it always has (between 80 to 90 all of the time, except the 100+ in stop and go)
Brake fluid level ok. Ac is working.

I have booked an appointment at M/B for next week, but was wondering if it is safe to drive the car at all with all aformentioned levels checked.

So, to my question, what am I leaking? Is it diff oil? SLS oil? Is there a way for me to tell for sure? I don't have tools nor skills, so DIM is out of the question.

Thank you in advance

Manu
1991 500E (ill, not diagnosed yet)
1963 impala SS (ill as well, bad month for me..