What type of refrigerant does a 1991 Mercedes Benz 300SL use?

The '91 used R-12. You need to buy the converters from Mercedes because the universal connectors are usually too small. The Mercedes converters are cheap. You can gety them from any Mercedes dealership. Make sure you vac out the system before you add R-134. Another good idea is to change the dryer and expansion valve before recharging the system.