I had a 1991 560SEL, which had a hydraulic component to the suspension system. Is it possible that the SL has a similar system?
Whether the 1989 did or not my first thought was a leaking shock absorber.
A friend drove a 1992 Toyota 4WD pickup truck 450,000 miles before giving it to a nephew. At 300,000 miles it started dripping clean oil on his immaculate driveway. Then it stopped. And 6 months later started again from the other side. Thats when he found the problem was the OE Bilstein shock absorbers.