The original refrigerant gas used was R12, this is still available, but is expensive and in short supply. Also, it can only be supplied/recovered by an EPA approved person. The 'newer' type gas was R134, which was made mandatory in 1994 for all new vehicles.
You can't charge an R12 system with the new type, without modifications. You'll need to replace all flare fittings with rubber o-ring type fittings (includes compressor, condensor, receiver/dryer and evaporator). You'll potentially need a new condensor, as the new gas isn't as good as R-12 in terms of heat release. Additionally, the old type oil used in the compressors as a lubricant, is not compatible with the new refrigerant, so you'll need to flush the system out completely to remove the old oil.
There is, an environmentally friendly alternative though. And no, I don't work for them, but there is a product called Hychill, which can be used in old car aircon systems with no retrofitting (new o-ring fittings, etc) required.
www.hychill.com.au
You will still need a vacuum pump to remove all the air/moisture out of the system though, before re-charging. If you do it yourself, you'll also need some pressure gauges etc to check that you've added the correct amount of gas, and the system is functioning as it should.
I'd highly recommend replacing the receiver/drier (do this every 5 years), and also the TX valve (located on the evaporator unit inside the car).
I replaced those parts, flushed the system in my GE Sigma, and despite the compressor being 30 years old, it works great, and blows cold air out the centre vents at around 10 degrees celcius.
For aircon info from the GE Sigma service manual, download here:
http://www.btscompany.com/Sigma_GE_Service_Manual
Download section 24.
John
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