After much reading of previous threads, I concluded that just about any oily-type liquid would work in a 240D's manual transmission, at least for a while, and that 90W was probably not a good idea for long-term use. The other threads mainly boiled down to Redline, a couple different grades of engine oil, and ATF. Out of curiosity, I called Mercedes Benz of Reno about an hour ago and asked what they suggest. The parts guy didn't know, so he went and asked the techs. He came back and told me to use 90W gear oil. Then about three minutes later he called back and said that he had told me wrong and that I should use Dexron III (aka regular cherry juice).

Since I don't know when my manual trans oil was last changed (though I suspect it was in 1993), I'm going to change it in the near future, and I will be using cherry juice.

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Whoever said there's nothing more expensive than a cheap Mercedes never had a cheap Jaguar.

83 300D Turbo with manual conversion, early W126 vented front rotors and H4 headlights 401,xxx miles
08 Suzuki GSX-R600 M4 Slip-on 26,xxx miles
88 Jaguar XJS V12 94,xxx miles. Work in progress.
99 Mazda Miata 183,xxx miles.