The Germans often refer to things a bit differently. When it comes to the OE stuff, I'm sure it is OK. When I say gear oil, I am referring to the more industry accepted definition of a heavy oil, and it is commonly found in weights, such as 75W80, 80W90, 75W145, etc. Those are heavy, thick oils, with a high surface tension. As such, they will not flow into the nooks and crannies of the shifting gearbox quickly enough, and will then fail to lubricate properly. Now, you will use those types of lube in the rear differential, but that is a completely different setup, with regards to the bearings and mechanisms inside.

The MB fluid is recommended for the SLK, and hence the Crossfire. I will still stand by and tell you that Mobil 1 High Mileage 5W20 or 5W30 will work just fine. The High Mileage (HM) is just a version of Mobil 1 (M1) that has a chemical makeup that lubricates better. It actually is the "old" M1 formula that still had ZDDP and sulphur in it, which has been removed from modern formulas of Mobil 1 for EPA requirements in engines. The High Mileage version lubricates better than the standard M1 you see on the shelf. You can use it in any engine (of the correct viscosity) without any ill effects, other than an oil that works better at being oil. Others have used more specialty fluids, like Synchromesh with good results. I found Torco to work better than Synchromesh in other Chrysler gearboxes, but it is also much more expensive. Motul also makes an MTF equivalent. All of these lighter oils will probably be just fine. If you have grinding issues, you may be using too 'thick' of an oil, or it is too cold where you are for that oil, or more simply, your synchronisers are wearing out and/or your clutch is not completely disengaging.

We went through a round here where heavy gear oil was being recommended, and it resulted in thrashed transmissions. That's what we want to avoid. So again, no 75Wxx, 80Wxx, 90Wxx ,etc.

Since people have had good luck with the MB approved fluid, that is probably your easiest/safest bet, if you still have questions about it. We have a great vendor (Needswings) who stocks it. Hard to beat that.