Changing oil, but what is this washer for?

[Joseph_H]

Please see the picture below where the line is pointed to a small washer that comes in the oil filter kit for my 95 E420. What is this washer for? there are three posibilities (or washers) that i see this washer can fit, but only one washer provided: 1) for the oil plug and I don't remove that since I use a topsider 2) for the outer washer in the filter housing cover and 3) for the inner washer of the filter housing cover (Note: there are two washers one outside the cover and one iside the cover for the filter housing bolt). The washer provided seems to fit all three mentioned above.... now which one is this for? Thanks.