...would it be a bad idea to convert an R134 system over? If so why? Is it even possible? Pros & cons? I've seen the opposite going from R12 to R134 but didn't find anything going back to R12 refrigerant.

That's a very intersting thought. I know in multiple states its illegal with out certain license and certifications. I've spoke to a few HVAC specialist who talk about going both ways and most say its not worth it along with the head ache of disposal or finding the other stuff. Now having that much it might be benefical, but I haven't heard of going that way, just the other way...

What would be the point? R12 is not better, it is just toxic. R12 is no longer in production and that makes it expensive. It is so expensive that it is cheaper to convert to R134a, no one converts the other way. I would sell it on eBay to those goofballs who do factory correct restorations.

different types of oil are required for the different refrigerants,
you would need to flush the system and replace the dryer.
sell the r12 on ebay, you can get $20+ dollars a pound.

$20 a pound?!? I remember back in my auto parts days we would sell it for something like 89 cents a can. People would come in and buy cases at a time. I even remember using a couple of cans for target practice with pretty cool results. My bad... sorry, ozone layer. I've done conversions to R134 but can't see any reason to convert to R12.

As others have mentioned, sell it on Fleabay.

The days of using r12 in automobile AC systems has all but disappeared.

It's a toxic chemical it can't be sold on Ebay, let alone shipped. I am an HVAC Mechanic for a large Bus company. R12 does get colder because of the denser properties of it. But converting from r134 to r12 is impossible with out replacing the whole system, so in other words it's pointless.

You're kidding right? It can be shipped and sold on ebay. Has to be sealed and marked but people do it all the time. It's not made anymore but not illegal to sell.
R12 R 12 Refrigerant Full SEALED Case of 12 14oz Cans 1 Free Oil Charge Can | eBay

In an sealed NEW package yes, but he has 20lbs of it probably in what use to be a 40lb tank of it. its unsealed and can potentially leak. there is a ton of extra paperwork and charges associated with shipping hazardous chemicals. it's illegal here in Michigan to sell R12 without a HVAC license(or so i was told in all my classes i had been through)

Could it be done? Sure. “All it takes is time and money.”

Pros:
• R12 has a greater heat transfer abilities than R134a. It will more quickly cool a given area to a lower temp than R134a.
• Because of the larger molecular size, R12 is more effectively contained by seals and gaskets. It doesn’t require barrier-style refrigerant hoses.
• Because you have 20 pounds of the refrigerant, you can support the system for the life of the vehicle.

Cons:
• R12 is NOT compatible with R134a. It requires dedicated gauges, recovery equipment and storage vessels. To ensure this, R12 equipment has ¼” flare fittings, while R134a gauges and equipment have ½” ACME fittings.
• Because of incompatibility, all A/C components must be changed out. Current-production evaporators and condensers can’t be completely flushed of all lubricant residue. The ‘left over’ synthetic-based oil will react with R12’s mineral-based oil and form a sludge, contaminating and destroying the compressor.
• Much more expensive per can/pound. Keep in mind that although the cans are called “pounds”, they in fact hold only 12 ounces. Not 16.
• This conversion will definitely lower the resale value of the vehicle. Because of the potential contamination issues, it won’t be supported by any repair center. At some point, the new owner will need to consider ‘reconverting’ the system back to R134a. Not only would this include all new components, but it would also include a fee for the cost off recovering the R12.

If you do your own wrenching and have the equipment, it’s certainly a possibility. Understand, your gains will be minimal. Although R12 is the more effective refrigerant, you can offset this with a larger, more efficient condenser. In fact, this is what car builders did as they transitioned from the serpentine design to multiport folded tube (MPFT) condensers. This new design is much more effective, and R134a refrigeration systems now offer similar cooling abilities. You’re basically going to a lot of effort and expense for little change.

The advice to sell it makes a lot of sense.

I'd just sell the R12 on ebay or craigs. I have some can left over from my Jeep Cherokee days that I need to sell off.

This is good info. Great first post too!

My original post was purely hypothetical. I don't have any R12 but I did see some for sale recently and it got me to thinking if I happened to already have the R12 or got it for next to nothing and had enough of it to "support the system for the life of the vehicle" (as you perfectly stated), would it be worthwhile? I think you gave me the answers I was looking for. In short — it could be done but not worth it since the effort would be huge and the payoff minimal at best.

If I did have R12 though, now I know it's best left for eBay or Craigslist for the people who still run R12 systems and need more.