[911nut]

Originally Posted by gizmofixer

Our cars are known to have 'porous' hoses and untraceable leaks.

R-134 runs higher pressures and is not as effective cooling.
You can convert to R-134 without changing any oil or the dryer.
Your tech just needs to be aware that the system will require more qty R-134 than the conversion charts show and the pressures will run correspondingly higher than a R-12 system.

The hoses in the 964 a/c system were engineered to use both R-12 and R-134a. They are not the old tech hoses that are designed to leak but are barrier hoses that operate under the higher pressures required to run R-134a.
If your car was delivered from the factory with R-12, there are 12 o-rings that will need to be changed out, as well as the receiver/drier and the fittings on the compressor.
R-134a has the ability to remove more thermal energy from the air than R-12. Usually the conversion process gets botched and improper operation is the result.
My opinion is that if you are going to spend the money to convert, go all the way. A receiver/dryer costs $30. Why wouldn't you replace it? It takes 10 minutes to drain the old oil out of the compressor. Why not do that?
Find a shop that has a reputation for working on Porsche a/c and work with them. If you can't find that, buy the "Without Guesswork" book, copy the R-134a High side pressure vs. ambient temperature graph and hand it to a good shop. If they are any good at all that's enough info to get a quality job done.