The oil is in the compressor. The vacuum removed moisture from the system, I think the moisture reduces the effectiveness and can lead to problems. The right amount of oil to add back in should be listed in the Factory Service Manual. I'm getting ready to convert my truck so I've been reading up. Supposedly the 2 oils (R12 and R134) are not compatible...I'm not sure why but I'm going to flush my compressor, change o-rings, and of course new drier. My condenser is busted, so new one of those for me as well.

83 4x4, 3" Lift, 33" Buckshot Mudders