I have a '97 E300D. I just replaced my spin on filter. I believe the bolt you speak of is called a banjo bolt. My filter came with two o-rings for this bolt, but my car only needs one of the o-rings so the other was thrown away. You should carefully inspect the condition of the oring at the bolt and the inner & outer (especially the inner) rubber gaskets on the spin-on filter.

You may want to purchase another spin-on filter and make sure you're getting the right part.

You should not need to tighten too much at all; overtightening can cause leaks. I tightened just enough to stop the leaking after I installed my new filter. Also, be sure to lubricate the outer gasket of the spin-on filter (just like you do for an oil filter).

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Neil
1997 E300D - naturally aspirated
280,000 miles (as of November 2015)