Yes, Redline differential fluid is good stuff and highly recommended.
You should easily be able to find it at any auto parts store. If not,
online sources like BMA parts, Baverian Autosport, and BMP carry it.
If you still can't find it, do a search for Redline Oil's website. It
should be able to direct you to a local auto part store.
Alternatively, any excellent fluid I'm using in my 90 E34 535i, 5spd,
104K miles, is Royal Purple. I recently tried it when I changed out my
trans and diff fluids. The shifting in my car is very, very smooth and
the differential is quiet.
At 60K, its time to change both your transmission and differential