What I am finding interesting is that on a 1976 W115 240D your oil level should not be high enough to leak out of that area. There is only supposed to be like 1.5 inches of oil at the bottom of the pump regulated by the oil drain tube on the engine side of the fuel injection pump.

That being said there is an O-ring in there some where that replacing may help.

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84 300D, 82 Volvo 244Gl Diesel