Here's how the 55's Oil Level Sensor Works ...

Conclusions:

The Oil Level can be as much as a quart above the "normal" level and the Oil Level Sensor (aka. Control System) will indicate "Engine oil level OK"

The Sensor does provide readings at 1.0, 1.5 and 2.0 quarts low, but not at 0.5 quarts low. (Consistent with the Owner's Manual, p 289.)

The Sensor may require the engine to be run before accurately updating to a new current status -- that was my experience.

Background:

I am going to be on the track this weekend with the 55 and I want to be sure that the oil level is at or above the nominal level, but below the "Reduce oil level" point. I'm not aware of how Mercedes designs it's oil pans, but BMW had (still has?) a problem with the oil pump on E36 M3s becoming oil-starved during long sweeping high-g turns like the Carousels at Sears Point and Road America. The oil pan was not baffled sufficiently to prevent the oil in the oil pan from sloshing away from the oil pump intake under these conditions. When on track with our M3s we always made sure that the oil was a quart above the "normal" level.

What I Did and What I Found:

I have 2000 miles on my SLK55 and the oil level indicates a normal "OK." I used the oil vacuum extractor (LiquiVac by Air Power America) that I bought from the USA MB Club to suck out about 2 quarts of oil from the oil pan via a plastic tube inserted into the metal tube at the back of the engine where the dipstick should be (a dummy plug is inserted in this tube.) As expected, the Control System (CS) indicated "Add 2.0 qts. to reach max.oil level."

I added a 0.25 qt. (= cup = 1/2 pt.) at a time and checked the CS display after each addition. After adding 0.25 quarts, the CS read "Add 1.5 quart".
I added 0.5 qt and the CS read "Add 1.0 qt." So far so good.

I then added another 0.25 qt (cup) and the CS read "OK". My conclusion is that the oil level can be as much as a quart low and the CS will read "OK."

I continued to add oil a cup at a time. The CS continued to read "OK" until I had added another 2.25 qt. at which time it read "Reduce Oil Level". I removed 0.50 qt, but the CS still read "Reduce Oil Level." But then I started the engine and waited for the 30 minute wait time. The CS now read "Engine Oil OK" and continues to display this "OK" message.

I believe that I now have the crankcase filled with between 0.75 and 1.0 qt of oil above the nominal level but still below the "excess" point.

Too bad that this is so difficult to determine. Can we find a dipstick that will fit into the check tube at the back of the engine, even if we do not keep it inserted while the engine is running? Perhaps someone with another MB model could check this. In the meantime, I'll look around to see what I can find. I've checked my BMW's dipstick, but the plastic piece at the end is too large to fit into the SLK's tube.

Also: Does anyone know what the display "Oil Lev --:--" is and how it can be used? This display appears when scrolling through the diagnostic displays (turn the key to the accessory position, press the reset button rapidly 3 times, scroll up to this display.) I was hoping that this would actually provide more info WRT the oil level, but it only displayed the above message at all times through this process.

Hope this helps.