I have done that on my previous 1990 300E. It leaked oil on the cap and a new one fixed that. The new one is also plastic but it does look a little different.

Then I ordered one for my 1991 300E just to feel safe. It is very inexpensive.

Speaking of caps, I have replaced the coolant expansion caps as well. About $5 each (every 80 - 100kmi) and makes me sleep better.