I noticed that my 993 was using more oil than I was expecting and proceeded to find the case of the issue. Luckily I have a lift within my garage, which enables me to look around the underside of the car. It was from here I noticed that oil had been seeping from the lower valve cover onto the heat exchanger and nicely coating the inside of the engine cover.

I did some research into this issue and found that it's a fairly common problem, something to do with the seals shrinking with age. I also found a number of people that had successfully managed to replace the seal themselves, but couldn't find anyone who could tell me if by removing the cover I would flood my garage floor with oil. So I decided to take the plunge and am very pleased with the result.

Note: While I have a lift which did make the job easier, I've read that this can be done, by jacking up the rear of the car and removing the wheel.
 

The problem. Oil has been seeping out for a while and the bolts are very corroded.

1. Make sure you can loosen all the bolts. There are 11 of them and they all easy to get too with the exception of 2, which are behind the heat exchanger pipe. If you can loosen them all then proceed to the next step, if not my advice would be to re-tighten them all and get the dealers to look at it. Rather they break a bolt in the engine, than you have that pain!

2. Remove the 3 spark plug wire and jam them in suitable places so they out the way.

3. Remove all the bolts and curse and swear a bit when trying to remove the 2 behind the exchanger. You need to have small hands and removing these 2 bolts alone can take > 15 minutes.

4. Once the bolts are removed, my cover didn't just fall of, but required some gentle persuasion with a rubber mallet.  Taking care not to apply too much hammering force so that you dent/mark the cover.  Get ready with a rag, since a small amount of oil will drip out when you finally get the cover off, although its not much.

5. The seal sits within a groove in the cover and can be removed with a flat screwdriver.

6. Clean the cover completely ensuring that special attention is paid to the seal groove. Using a little engine oil, gentle oil the new seal and press into the groove of the cover. It will only go in one way due to its interesting shape.

7. Apply a little more oil to the seal edges and reposition the cover back on the engine and secure using the new bolts. It's worth using new bolts since they look nicer, probably give a better seal and only cost a few pennies/cents each.

8. I've read a number of comments about how tight these bolts should be. The idea is that they need to be tight enough to form a seal, but not too tight to overly compress the new seal. Since everyone has a different opinion of what is tight, I found that if you use an allen key held between your fingers to tighten them until your unable to tighten further, then give them a further ¼ turn whilst holding the key in you hand, you should be fine. When you're finished the bolts will be tight, but won't be torque up to the hills! Remembering that you can always tighten them further if you get a small leak, but will need to replace the seal again if you over tighten it.

9. Reconnect the spark plug wire and congratulate yourself on a job well done!
 

The finished job!

 

Here is a very terse procedure for changing lower valve cover gaskets on a 993- no need for photos on this one: 

If you see oil leaking from valve covers on your 993, the most probable cause is the gaskets, which harden up and dry out after 7 or 8 years. 
To replace, follow this procedure: 

1. buy new gaskets from your favorite parts source - you need the lower gaskets, a total of two (left and right are identical) 

2. remove shield under engine, muffler tips and mufflers just as if you were going to change the spark plugs. In fact, this job is best done at the 
same time as you are doing a major tuneup because you have to do over half the work anyway. 

3. remove the rubber heater hoses that connect to the heater box - to do this loosen a hose clamp at the top and bottom of the rubber hose. 

4. Optional (but recommended) remove the muffler support bracket on both sides - there is a bolt with a 13 mm head holding the bottom, and a cap screw with a 6 mm allen screw holding the top.  With a 5mm allen wrench, remove 10 hex capscrews holding the cover in place. One screw (the rear most screw on the bottom on both sides) is nearly invisible and must be removed with an "L" shaped allen wrench. The others 
can be removed using a 1/4 inch drive socket and a high quality 5 mm allen head socket - I recommend the one made by Snap-On. Don't use a cheap wrench, if you damage the allen screws it will take you forever to get them out. Buy a good wrench that fits. 

5. remove valve cover,, being prepared for a little oil to drain out (maybe 1/4 to 1 cup of oil) pull out old gasket, put in new gasket - it only 
fits one way, so don't force it in backwards or upside down.  

6. wipe a little oil across the face of the new seal to lubricate it (optional) and put it back in place.  

7. reinstall the 10 cap screws. no need to make them super tight, just tighten until they seat snugly. 

8. repeat with other side of engine. 

9. using a rag, towel, or your neighbor's cat, wipe all the oil that has accumulated ontop of the heater boxes off 

10. reinstall heater hoses, mufflers, tail pipe extensions 

11. reinstall shield under engine