Automotive Air Conditioning Information Forum

I’m recharging a 1992 Ford F-250 Diesel with R12. I had a Ex Ford Pro Mech do it a couple of years ago and it was a slow waste of refrigerant. I’m replacing the condenser (leaking) and accumulator and orifice tube. My questions are:
Oil is Mineral (Ester)??
How do I determine how much oil to add ...