I recently had my discharge hose replaced because of a leak, and they said that since it's an old ('89) car, the stick system would be R12 and it would need to be converted since R12 is now illegal. They did a flush of the whole system and replaced the oil and filled it with 134-a refrigerant.

The oil they used was Suniso 5GS oil. It's mineral oil and will slowly break the compressor if used with 134-a. Here's a quote from the site:
"SUNISO GS Oils are premium quality oils expressly designed for use as refrigeration compressor lubricants.

SUNISO GS Oils are miscible well with HCFC and CFC refrigerants such as R-22, R-502 and R-12"
http://www.sunoco.co.jp/english/product/gs/index.html