I appreciate all your thoughts on the subject. I believe now that I will convert to 134A when I need to. Currently, my 1989 300SE is cooling pretty well--I found my last can of R12 and topped off with that, and it brought duct temps down to a reasonable level. I had converted my other MB, a '79 300TD, several years ago, and it cools pretty well. And of course, the 134A is readily available in recharge cans. Again, thanks to all who took the time to post.

Joe B.