^ its easy. Smelly...but easy.
General rule, if you don't know...then it has never been changed most likely.
How much is in there?
There is only about 8oz. Two 4oz. bottles of the GM supercharger oil is all you need to fill it. Some times more, sometimes less. Any GM dealership should have this in stock most the time. Its usually about $7.00 or $8.00 a bottle.
You have the right idea, take out, or in your soon-to-be-case

SUCK out all you can, and refill to the bottom of the threads where the plug goes. Some times direct sun light or a flash light is a good aid in this to see down in there good. You cannot really over fill it, but you can under fill it. As long as its to the bottom of the threads of the fill hole, your good.
Comes out pretty smelly, so don't get it on you or your cloths. Normal color is dark ice tea looking color to black at times. But it goes in perfectly clear brand new from the bottle.
What I do is suck it out, and transfer it to another bottle that has a screw on seal able lid to store the oil in. I use old rubbing alcohol bottles myself. At any given time, i could have about 3 or 4 of them filled up.
~F~