Changing your rear differential fluid is a very essential part of your car maintenance. Have a habit of changing your rear differential fluid every 30,000 miles; change it earlier if you track your car, or if you have a frequent habit of pushing your car harder during daily commutes. I highly recommend having some essential tools as shown in the video, especially the fluid pump. You don't need to buy this exact one, but having one will speed up the process; even with my simple to use fluid pump, the Wilmar W1139 Fluid Pump, it took me around ten full minutes to pump to the fluid limit; my arms were very sore by the end of it.
There are a couple of choices when it comes to choosing your rear differential fluid, but I've heard only good things on Motul gear oils, so I gave it a shot. I have been driving with the changed fluid and have had no problems at all; my rear differential seems to be liking this fluid.
A quick note: Depending on the year of your STI, you may not be able to use this gear oil, so please do your research before purchasing.
The whole process should really only take you an hour or two, including set up and clean up times. However, this isn't a competition, so take your time and be careful.