Hi just to add to this thread I have been running R12a in my 300E for the past year with no problems .R12a is a mix of butane and propane ,down here in Puerto Rico its legal and cheap.
My system was converted to R134 but was never very cool the change to R12a dropped the temp at the vents by 20 degrees!!
Anyone interested should do a search on the web for info on R12a.
I plan changing the gas in my 420sel when the AC needs work . Steve.