I thought I'd jump back in here as we've decided to add the bypass filtration to all of our cars, the 300SD included. My oil pressure sending unit needed to be replaced anyway, so I've been trying to find the proper fitting. Amsoil has a fitting for Japanese cars which has the right thread for me (12MM x 1.5MM). I have ordered it and HOPE it's right. I have been able to find parts that were close, but not quite the right thread. I went to the auto parts store and looked at Trailer Brake Metric Tees (see following link for a description, etc.) -
Stant catalog of fittings
On the return line, Lightman, I'll have to take a look. I have also heard of people tapping into the oil filler cap. That certainly seems like the easiest, but least elegant, way to do it. But tapping into the valve cover could allow it to be unobtrusive.
I'll let you know how things go.