Personally, im not a big fan of "replacement" refrigerants. If my system was R134a, chances are that I would just recharge with that. R12 is still available if you know where to look, even though you need that EPA certification to get it. Expensive, yes, but it's not worth the risk of possibly mucking up your system down the road with a blended refrigerant.

Of course, this is one man's opinion.