My 1988 560SEC A/C is cooling poorly this year after 6 years of perfection, and I am ready to convert it to R134A. What is the proper methodology? I have a Snap-on vacuum pump and all guages and fittings to attach the system, as well as a recovery unit available. I just need to know if I should replace the receiver-dryer at this time and what to do before vacuuming the system down and recharging, such as precisely how much oil to add and what brand is recommended. Also, how many ounces of R134A is required in the system and what should the high side and low side guages read for a perfectly charged system.

Thanks!