Mystery Part from Stihl Chainsaw
The rubber gasket is a vibration isolation mechanism, not a liquid holding mechanism, based on experience with the parts of my Stihl saw (different model but those parts are similar.) All the mounting points between the handle/frame and the saw motor use those rubber-mount nuts.
While lacking a convenient exploded parts diagram, the manual I found does have a list of things to check when the bar is not getting enough oil.
https://stihl.partsmartweb.com/Catdata/EMP_IMG/STGSTH/Images/SERVICE/00590215.PDF