M-B used to sell just the gasket so that you could replace that only. Now, as you say, they sell you a new cap - though it is fairly inexpensive.

As I understand it, maintenance schedules that dealers use call for replacing this thing periodically (don't know the interval). My practice is to replace it as soon as I spot it leaking - this is easy because I check the oil every time I put diesel in the tank.

Good luck-

Terry