Here is what I do and I believe most A/C folks use this as a rule of thumb.
Pour out the old oil that you are replacing into a container and measure it.
Put back in new oil in the same amount.  You are never going to get all the
old oil out and overfilling with oil could, at the least, just take up space
that could be occupied by refrigerant (making the system cooler).  Please
note that new compressors come full of oil unless otherwise stated on the
box.  Just FYI.

Hope this helps,
Ken Wilford
http://www.vanagain.com
John 3:16
Phone: (856)-765-1583
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