Well dang, I guess that buick I converted is running on over 4 years of borrowed time then!

Bottom line - you have to use stuff that's compatible. R12 and R134 use different oils. You wouldn't put ATF in your engine and expect it to run just as before would ya? There's a simple fix for that though... drain the oil! I don't really see it as a big deal. Heck if the systems not been changed/charged in it's lifetime it's the original oil yet - how's that for a oil change interval. The oil is cheap, and it's not really that hard to drain it. R12 systems run a mineral oil. R134 systems run an Ester or PAG oil.

Accumulator (receiver dryer) Ok so they're like 50 bucks or so. Yeah it costs enough, but that thing's been zapping moisture out of the a/c system for 20 years How's that 20 year old air filter working out for ya? R134 uses a different dessicant inside than the R12 systems R134 systems use an XH7 or XH9 dessicant and is backwards compatible with R12. R12 systems use an XH5 dessicant and is not R134 compatible.

Those 2 items are darn near just maintenance items on vehicles as old as ours! After you knock out those two issues you're just a few o-rings shy of a full R134 conversion. Likely the "black death" is due to people charging them with R134 without changing to the proper oil or swapping out the accumulator for a compatible model..

And why would an a/c shop ever say "Oh yeah that stuff I charge an arm and a leg for you can do yourself at home in 20 minutes for 30 bucks"