I have seen some that will look red on your finger. I don't think it's critical exactly what Hypoid oil you use as long as it's Hypoid gear oil and not something else.
It's plenty simple and easy enough to change it. Since you don't know how long it's been in there it wouldn't hurt.
Plain old 85W90 or 90W will work fine. I got a five gallon bucket of stuff many years ago, when my 240D was young. I've changed it about every 100,000 miles and have had zero problems with the rear axle or suspension in any way.
The only reason you would have to be picky about the lubricant, is if it is a limited slip.
To answer the original post in this thread, I don't know the exact quantity, but it's not much. The main thing is to just drain it, and fill it back up 'til it runs over. The suggestion to remove the fill plug before the drain plug, is a very good statement of wisdom.