This is only a wild guess, but I have read in the forum that theres a thermostat in or around the oil filter housing that allows oil to flow to the oil cooler after a certain temperature on the 124 cars, this thermostat could be sticking open and allowing the filter housing to drain completely, again just a wild guess but it might point you in the right direction. Another thing I noticed is at least here in NYC these fast lube places have really badly trained people working on cars, the other day one of them tried to fit a filter from another car in my car. The one he had was clearly taller and he had already put the cap on and was forcing the nuts in when I noticed so I told him to take it back out and the filter he had put in there was crushed, now imagine what that crushed filter would've done to my engine and how well it would've fit in there.

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87 300d Turbo 241K with original #14 head (blue)
87 300d Turbo 198K will run great again someday (silver)
84 300sd sold ( buyer drove it to Paraguay )