Just wanted to follow up and let folks know what I experienced with my 99 SLK differential fluid change. I used Royal Purple and after 3-4 days I was getting leaks around the seals as I drove. Checked the fill and drain plugs as thought maybe I had not done a good job putting them back. No - they were all tight.
So I called the parts guy at the local dealership and asked him if they had the Mercedes gear lube for a 99 SLK differential. He said even they don't use Mercedes fluid for differentials, they use NAPA part SL2472, STA - LUBE NAPA AUTO PARTS
which is a 80w-90w GL-5 gear lube that cost about $11.00 per qt (after my Vet discount).
Four days of driving with the NAPA STA-LUBE and no leaks - hard to believe my SLK did not like drinking Royal Purple.
I also had this happen on my 2001 Harley, used Red Line Gear oil and started to get leaks around the gaskets after 3-4 days - changed back to regular gear oil and no leaks.
Anyways, your milage my vary and this is just my experience - you should always use authorized Mercedes products that cost lots of money to avoid any issues - this is not advise (legal disclaimer).