Many of the drain/fill plugs seem to be 10mm hex (allen) type- at least those that I've seen lately (and that's been quite a few!!).
To get the stuff in, I have had pretty good luck using a plastic pump with a ~2ft clear plastic tube attached to the end. I think I got mine from Autozone or Pep Boys. It's designed to screw into one of those short rectangular gear oil bottles, I've got an old one that I use- I just keep filling it with whatever oil I need to pump into the car (diff, tranny, etc).
I have not actually tried it this way, but you might be able to use a LONG piece of hose on the redline bottle (or a long hose + funnel), and hold the funnel up near the rear wheel well, then fish the hose down over the top of the rear tire, and into the diff fill hole. I've done something similar to this when filling manual tranny's in E30's as well as E21's and 2002's- basically run the hose up to the engine bay- near the firewall, and into the fill hole. It REALLY helps to have an assistant for this method.
Bret.