Your '87 300 D will never cool as well with R-134a, unless you perform additional modifications.
Also, ND compressors generally last about 150K miles. If the vehicle is still using its original compressor, you'll want to seriously consider renewing it at this point, esp. if the system has been run for a period of time without a full oil charge or if you wish to switch refrigerants. R-134a runs at a substantially higher discharge pressure than R-12 and this will stress a worn compressor.