I"m going to disagree with Z, I don't do that often, but this time I think I must.

From Benzworld:

The 4 red arrows indicate the o-rings everyone changes. The stem and the oil filter cap are detatchable but hard to reassemble. There is a valve located where the 2 parts detach (pink arrow) it has a o-ring on it (green arrow). That is the one you have left over.



Over the course of 12 years, 120,000 miles and three Roadsters, I've replaced that o-ring one time. That arrow looks violet to me, not pink.