I changed out my transmission fluids at 28,000 miles to Mobile 1 synthetic 75W90 gear oil. The immediate results of this was a much smoother shifting action.  It was well worth the investment.  The skill level of this DIY from a skill level of 1-10 (10 being the hardest) is a 5.  The only special items you will need besides 4 quarts of gear oil and new washers would be about 5 feet of 3/4" or 7/8" clear vinyl tubing.  I also recommend you get a good quality 10 mm hex socket which can be mounted on a ratchet instead of using an allen wrench. 

(1). Jack the car up on 4 jack stands, please refer to "How to jack your car up" section of my DIY.  Remove the engine undertry and the transmission undertry.  The engine undertray is held on by philips screws, and the transmission undertry need to be pulled out toward the rear of the car once the philips screws are loosen. (Not sure if it is necessary to remove the engine undertry to do this DIY.)

(2). First remove the filler plug, per Ray Calvo's recommendation it is safer for you to remove the filler plug first just in case you drained all the fluid and stripped the filler plug while trying to remove it later.  At this time you will also need to place a drain pan underneath since the transmission fluid will be purring out of the filler plug as soon as you remove it.   Jeff Chan recommends that just to loosen the filler plug first without completely removing, since fluids came pouring out of his filler plug. 

(3)  Loosen the drain plug and drain the transmission fluid.  The transmission drain plug is a magnetic plug that will pick up what ever metal shaving in your transmission during normal driving and shifting.  If there is an excess amount of  metal shavings found on your drain plug I would consult a transmission specialist first before you clean the plug up and put it back on the transmission.  Here are two pictures that shows before and after pictures of the magnetic plug.  The condition of my drain plug at 28,000 miles was normal. Replace the washer and install the drain plug.

(4). Stick the vinyl plastic tube in the filler plug, run the tube through the right side wheel well opening.

(5). Pour 4 quarts of your favorite transmission fluid in the tube slowly and pull out the vinyl tube from the filler hole and install the filler plug with a new washer.  You will need to install the plug quickly, since new fluids will come purring out of the filler plug once you remove the vinyl tubing. 

You are done!   Go for a test drive you will love it!

Important fluid information update May, 2000

As the winter months came the Mobil 1 transmission fluid developed a hard shifting problem in my transmission.  This problem only occurs when the car is cold and the outside temperature is around 35-45 degrees.  During the warm up period I had a terrible time shifting into 1st gear and felt a big bump when shifting to 2nd gear.  But if I double clutch it before shifting to 2nd gear the bump feeling is eliminated.  So the conclusion was that the Mobil 1 fluids were too thick and when it was cold it was not flowing off the syncros fast enough for the sycros to do its work.  But as the transmission warmed up the hard shifting problem went away.  After talking with several Porsche technicians I was told to switch back to good old Dino fluids to see if the situation would improve.  So I drained the fluids out and poured in some Valvoline Dino (fossil) 75W90 gear oil and it did help improve the hard shifting issue when the car was cold, but once warmed up it seems to require more effort for me to shift through all the gears just like before I switched over to Mobil 1 synthetic oil.  I was not satisfied with the result and after talking with someone from "Red line oil" who owns a 993 C4S and had good luck  using a mixture of 3 quart 75w90 and 1 quart 75w80 red line synthetic fluids I too tried this combination.  It did solve some of the problem, but again it is still not as perfect as before when it is cold.  I then recently switched to Swepco 201 gear oil, it is also a 75w90 spec. transmission fluid, but not synthetic.  The Swepco 201s was the thickest of them all, and it made the transmission feel very different.  Almost like a buffer kind of feeling.  I am still having trouble shifting to 1st gear when the transmission is cold, unless I get the car to a complete stop for several seconds I will have trouble put the car in first gear.  But if I shift slowly to 2nd gear when it is cold I can give enough time for the syncros to work and eliminate the grinding sound.   So this is what I am sticking to for now, will see what happens.