Mineral oil based GL-5s are typically 80W-90 and synthetics are typically 75W-90. Either is okay. The only other somewhat common GL-5 is 140 and you DO NOT want that.
The most important issue with a limited slip diff. is to be sure that the gear oil you use either has the friction modifier additive or if not, you must buy a the friction modifier and add it.
I've been through this many times with GM limited slips units that have the same clutch pack type (or cone type) limited slip and I ONLY use GM oil and additive because I know it works.
I've heard that Redline 75W-90 gear oil works okay in limited slips and Redline says it already has the additive, but most commercial GL-5s don't. Redline also has a 75W-90NS (non-slip) product for open diffs.
If you want to use oil you buy from an M-B dealer you should determine whether or not the product you use has has the friction modifier for limited slips or if it must be added.
If the M-B product is specific to the 16V and 560SEL then is might already have the additive as these were the only two U.S. models from that era that had a limited slip.