I called the MBUSA 800 (1-800-for-MERC) number with that very question when I was shopping for a new used car; the guy said that '92 and newer have the 134a refrigerant. they are amazingly helpful and knowledgeable at that number.
RIP "Betsy" (Oo=*=oO)
'96 E 3 2 0 (W 2 1 0) M 1 0 4 (Totalled)
Spaces in sig so as not to screw up the SEARCH; every time someone searches for that MB they don't want my sig!
2004 Audi A8L
'98 VW Passat 1.8T 5M
'87 Alfa Romeo Milano 2.5L 5M
'67 Impala convertible, 327cid