If after checking pressures you need a top-off, use the right stuff.

I too have converted to R134 in my '83 coupe and now wish I never had. It will never be as efficient as the original R12 even though I changed compressor, expansion valve and all hoses and o-rings. I think the only thing that can make R134 work well in our old dogs is a good efficient cross-flow condenser. Maybe some day.