This DIY project in the skill level of 1-10 (10 being the hardest) it is a 6.  To increase the download time I will only post minimal numbers of pictures and the rest can still be access by click on the underlined links on the word. 

Some of the most important tools and items needed for this DIY is a oil filter socket, and over sized drain pan.

There are two oil filter in the 993 engine, Porsche added an additional oil filter on the 993 engine to provide extra filtration of the oil for the protection of the hydraulic lifters. 

Parts you will need to replace on this DIY:
About 11 quarts of Mobile 1 oil, (Or the oil of your choice)
900 123 118 30 Seal washer for oil tank drain plug, 22x27mm 
900 123 118 30 Seal washer for engine drain plug, 22x27mm (thru '95) 
900 123 095 30 Seal washer for engine drain plug, 21x26mm ('96 on)
999 701 269 40 O-ring for oil tank drain plug, 11X2.5 mm 
993 107 203 02 Engine oil filter
993 207 201 01 Oil tank oil filter

Tools needed for this DIY:

15 mm 6 point socket (can damage drain plugs with 12 point)
15 mm box end wrench (6 point preferable, if you can find one)
13 mm socket
10 mm socket
Ratchet
Large flat-head screwdriver
Medium Phillips screwdriver
Oil filter wrench 
Hydraulic floor jack
Jack stand
Torque wrench
Large oil pan

(1). Jack your car up on 4 jack stands,  please refer to "How to jack your car up" section for details.  Remove the right rear wheel.

(2). Remove the 4 philips screws that holds the plastic panel on the bottom right side of the car.  And unbolt the 10 mm plastic bolts that holds the entire panel on the bottom right side of the car.  Pull the cover off around the jack stand position area, and pull down the side panel, you will not need to remove the panel completely to access the oil tank drain plug and oil tank filter.

(3). Remove 1 10 mm plastic bolt in the rear wheel well area.  Pull the wheel well panel back to gain access to the oil tank filter.

(4). Place the large oil pan underneath the oil tank drain plug and unbolt the drain plug.  Remove the oil tank filter through the rear wheel well panel opening that has been pull back.  After you have drained the oil from the oil tank, install new filter (long one).  Replace the washer on the oil tank drain plug, install the drain plug tighten to 40 ft lb of torque.  It is a good practice to check if the rubber seal has fallen off the old oil filter and are stuck on the engine before you install the new one.  This used to happen a lot back in the old days, sometime the new filter are installed with the old oil rubber seal still stuck on the engine case, we call it "Double gasket".  Leaking will occur when that happens.  Also when installing the new filter make sure you don't over tighten. Since you will be using the oil filter socket you will tend to over tighten the filter.  Remember they were suppose to only be hand tighten.

(5). Remove the engine undertray and the transmission undertray.   Dzus style fasteners; ¼ turn unlocks and releases the screws that holds the engine undertray up.  For the transmission undertray you will need to slide the panel out toward the rear of the car. 

(6). Place the oil pan under the engine drain plug, and unbolt the drain plug to drain the oil from the crank case.  When complete replace the washer and bolt the drain plug back on at 40 ft lb of torque.

(6). Remove the right side ventilation rubber hose and "r" shaped ventilation door.  There are a total of three straps you will need to loosen.  The rubber hose connects up, the picture on the right shows where the strap needs to be loosen to remove the rubber hose.

(7). Unbolt the oil return line under the second small oil filter.  In order to remove this filter you will need to move the oil return line out a way.  (A trick told by a Porsche mechanic)  It is possible to remove the oil filter without moving this tube, but it is easier this way.  The left picture shows how the oil return line blocks the oil filter from dropping straight down.  There is only one bolt that holds the oil return line on. (center picture) Place  the oil pan underneath this line before it is pull out of the engine case, oil will come purring out of this line once it is pulled out.  In order to remove the oil return line out from the engine case you will need to use a flat screw driver slowly and carefully pry out the oil return line from the engine block once the bolt is removed. (The right picture shows how to pry it out) 

This picture shows the oil return line removed. You can also see the oil filter wrench already on the engine oil filter.
(Contributed by Jeff Chan)

(8).  Remove the small filter and replace it with a new one.  Remember to apply a small amount of oil to the oil filter seal, make sure that you haven't "Double Gasket" 'ed, and to hand tighten only. 

(9).  When reinstalling the oil return line, you may have problems with getting the bolt hole to match up properly. The oil return line is metal, however, if you follow it up about 8 inches, there is a rubber connector section. You can use massage this rubber section to get the bolt hole to line up with the crankcase. In this picture, I have shaded the rubber connector portion in red 

Note: The service manuals recommend changing the o rings for the oil return pipe when ever you take out the oil return pipe. There are a total of three o-rings that needs to be changed.   999.707.316.40 x 2 and 999.707.309.40 x 1 (at 50,000 miles I still haven't replaced them once yet)

(10). And reinstall the rubber ventilation tube and hose. 

Pour 9 quarts of oil in first, start the engine check the oil level and pour more if needed.  I usually end up pouring 10.5 quarts of Mobil 1.  The pouring process gets back up once you get about 8 quarts in the oil tank, you will need to slow down the pouring process or back up will occur.......

If you followed the above procedure exactly you will need to put in 10.5 quarts of oil.  This should save you sometime to figure out how many quarts of oil to put in.  With 10.5 quarts the needle for oil tank level gauge on tha dash will point at 3:00 colock position once the car is fully warmed up and oil thermostate open. 

During this DIY I broke a new personal record in changing the oil, 1 hours including the time spent to take these pictures. 

Additional information For 993 Twin turbo oil changes:
You do not have to loosen the rocker panel to access the oil tank drain plug on the twin turbo model 993. There is a small access panel on the bottom of the rocker panel, close to the rear wheel, fastened by two phillips screws, which allows you to access the oil tank drain plug properly. 

Each turbo contains a small oil resevoir. They should be drained at this time. Be sure to replace the washer when reinstalling the drain plugs. 

When filling the oil use a funnel will help you avoid spilling in the engine compartment. Also, if you have a turbo, you can remove the intercooler shroud to get better access to the oil filler pipe. 

There were some talks recently about the Mobil 1 Euro grade 0w40 oil.  So I was curious and purchased 12 quarts of the stuff from my local Porsche dealer (the only place that seems to carry them at this time)  At a whopping rate of $6.26/quart.  Just to fill you in on the complete story, I usually change to a lighter weight Mobil 1 oil (Green label bottle) right before the winter months, and change back to the 15w50 oil right before Summer.   I thought this would be a good opportunity for me to try out the 0w40 weight oil to see how it performs.