I wouldn't worry about losing capacity if you should convert to R-134 in the future. I personally converted an '84 300TD dark blue wagon to R-134 and have a '91 300TE wagon (also dark blue) that had already been converted. They both cool just fine here in Mississippi where the temperature and humidity hang around 95 in the summer. The conversion is more questionable on cars that have capillary tubes instead of expansion valves and undersized condensors. The trick (as usual) is to make sure whoever does it knows what they are doing as the R-12 uses mineral oil as lubricant and R-134 uses ester-based oil. This requires that you totally flush system to remove old oil.