Old car has R-12, very expensive and difficult to get now.

You can buy a kit (Wal-Mart, about $30) to convert it to R-134a, which is cheap and more environmentally friendly.

The major issue is how to convert. Technically and officially, you need to suck all remaining R-12 out of the old system, then put the kit adapter on, charge the system with R-134a. Almost impossible for home DIY.

Practically, I know people simply put on the kit and charge, with R-12 still in the system. Some people might say it is illegal, but a lot of illegal things are happening around.