The BMW E34 Air conditioning circuits are filled with either
R12 (1989 - 1992) or R134a (1993 - on).
See A/C upgrade
for more information about switching from R12 to R134a. PDF
File for Condenser exhange.
The A/C system is split into two sides, a high pressure side
and a low pressure side.
You need to discharge your circuit for any work on the A/C
system. Local A/C shop will give you a credit if the R-12
is recovered. I don't recommend to evacuate the circuit yourself
with a vacuum pump because of the environment concerns.