My books all say ATF in the Manual Trans, I however don't personally believe it matters between a good grade of motor oil, or ATF. I do believe however that 90 weight gear oil is too heavy for the application. Clearances for this weight were not designed in by the manufacturer.

On the fuel leak, dollars to donuts you have a fuel return line, or the plug on the last injector leaking. Do yourself a favor and replace all these little devils, including the one piece to the filter. Also a good autoparts store will carry replacement line that is far superior to the original woven outer layer line. And carry a spare plug for that last injector to avoid unexpected diesel fuel smell, unless you really enjoy it.

Check after cleaning, the oil filter adapter to block junction for your leak, I also have seen a couple of oil filter housings with leaks at the inner stud that holds on the top cover. Check oil pressure guage line connection at filter also.

I agree, 123 manual trans Mercedes is a Masterpiece of Engineering. I worked on them from the time they came out in 1977 until I retired in 1993. I was a registered technician with Mercedes Benz of North America, as well as owner of my own shop for several years.

I have had my 115 1976 240D since new, it also is a manual shift. I have ran it Turbocharged (my own installation) for the past 15 years or so, with no ill effects.

Rob