David,

If you want to simply perform an evacuate/recharge on your 300SD, I would suggest at least replacing the receiver/drier and switches, of course since you will be emptying the system you might consider going ahead with a new expansion valve as well. Anyhow, if your car has 134a in it, you will only recharge the system with 80% of the original R-12 capacity. By a rule of thumb, 2 ounces of oil should be added anytime a recharge is done but it really depends on how much you recover from your car's A/C system on the recovery cycle with the machine. I would set the vacuum to run for at least 1/2 hour and make sure it holds at least 30 in/Hg for no less than 10 minutes AFTER the vacuum operation is complete. Add the oil while system is under vacuum and then begin charge operation. As for which oil to use, we have had excellent results with Castrol PAG 46 which is a synthetic compressor oil and already has UV tracer dye in it. Roughly $8 for an 8 ounce bottle.

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Regards,
Aaron