refill a/c

hi everybody,

yesterday i realized that the a/c on my E320 cdi (diesel) W210 is blowing warm air. the car is only 4 years old, so it's kind of early for the a/c freeon to be empty, but this would be my first guess.

so i was wondering if anybody of you has experience on refilling the a/c on a W210?
i used to refill the a/c wit R134a on my 94 GC jeep without any problems.

thaks a lot in advance!