1975 240D oil filter and circulation problem
 

My daughter bought a 1975 Mercedes 240D several months ago and we just tried to change the oil. The oil was very dirty and it took about 5 quarts to refill it. The problem we confronted was when we tried to insert the oil filter from NAPA. First, the oil filter was too large to fit into the canister in the car. (about 3.75 inches and the canister was about 3.5 inches). The oil filters from Rock auto vary in size from 3.54 to 3.75...very strange. worse yet, there was no oil filter in the canister. It did have oil in it but it was not dirty and there was no evidence that it was circulating. Is there a check valve or other place it could be obstructed so that there is no circulation in the oil filter canister? (I'm going to call the maintenance facility tomorrow that was supposed to be maintaining this car for the last several years.)
Any suggestions will be appreciated.

Assuming it is like my 220D, there should be two filters in there. A white plastic prefilter and a regular canister filter. Without the prefilter, no oil gets filtered, I believe. Check out:
http://www.peachparts.com/shopforum/diesel-discussion/212929-pre-filter-nylon-screen-220d.html
http://www.peachparts.com/shopforum/diesel-discussion/256551-oil-filter-220d.html
http://www.peachparts.com/shopforum/diesel-discussion/256622-72-220d-plastic-oil-filter.html#post2245076

I have only used online vendors for parts (like our hosts), so I don't know if Napa carries the right filter. Sorry to not be able to help there.