The '75 has a funky pre screen filter in the oil filter housing. Make sure it's there. If it was ever taken to one of those crappy 15 minute oil change places chances are they threw it out which would be a bad thing. Here's a link to the oil filter housing schematics (item 224 on top schematic).

http://www.detali.ru/cat/oem_mb2.asp?TP=1&F=115117&M=616%2E916&GA=722%2E108&GM=716%2E002003&L=760%2E101++++++++++++++765%2E700&CT=M&cat=004&SID=18&SGR=045&SGN=02

If it's not there you can pick one up at the following URL:

http://www.mercedes-benz-parts-direct.com/

OIL SCREEN
Top - Nylon Screen
000 184 57 25
Oil Filter
Top - Nylon Screen MBZ005209
Mahle
$19.96
EACH