Possibly because it's dripping off of something when on an incline that it's otherwise puddling up on when it's level. I'm not at all familiar with the 560SL - does it have the heavy plastic noise shield bolted on under the engine compartment? If so, that's a perfect place for oil to collect. When you're driving, it's probably being blown back and off, and when you park on a level it's probably just collecting on the shield. When you park on an uphill incline, it will run out the back of the sheild and drip on the ground. When you park on a downhill incline, it just runs forward and collects at the front of the shield, which curves up to hook behind the front bumper cover and leaves no opening for the oil to drip out of (at least on my '86 300E that's the case).

If this is the case, just remove the shield for a few days and observe the ground under the car for the location of the drips. It should probably drip even on level ground then, and hopefully you can trace it back to the source.

Gary