A few years ago I was changing the oil in the transmission in my '82 300d. It's a German-spec Getrag 5-spd. In the owner's manual it made a big hoo-hah about how I shouldn't never ever use anything but the original and absolutely genuine (accept no other! Beware substitutes!) gearbox oil. Well.
So I went to my Mercedes dealer, and they spent a lot of time looking at their computer screen, then the micro fiche, then the dog-eared old paper. Nothing. Muttered conferences, nasty sideways looks. What is this hayseed trying to pull over on us, anyway?
So finally they get on the phone to Mercedes US, and after exchanging secret passwords and fraternity handclasps over the phone, the Head Parts Woman comes out with a piece of paper with a parts number on it. "Mercedes US doesn't have this product," he said "Take this to a Chevrolet dealer, and they can help you." with that he pointedly turned away and tended to his next customer.
At the GM dealer, they wondered where I got the number from, and then promptly came up with the oil. Out of curiousity I asked if that was the oil for the 6-speed manual box in a corvette. He checked the number and said it was. "How'd you know that?" he asked. I said that I'd read somewhere that it was a Gertrag box, and sort of put two and two together.
I don't know what this story indicates.