For future reference, shop air is "ok", but the best choice is nitrogen, because it is inert. Compressed air contains small amounts of water (which is why the tank as a drain on it), which is bad for your system regardless of which gas you use.

The key to removing all moisture from your system is to pull a deep vacuum for a long time. The longer the vacuum time, the more molecules of water boil off and get sucked out. Most cheap automotive vacuum pumps are single stage and pull a very weak vacuum, which necessitates a very long vacuum time. I use a robinaire dual stage for all of my A/C work and pull down the system for about 30 minutes.

Like others have said, use only 134-a or r-12. There are dozens of refrigerants on the market, but only those two are suitable for automotive use.

Also, if you're going to switch to 134-a, you should replace the original rubber hoses. R-12 molecules are larger and 134-a can 'seep' through the old hoses.

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A.S.E Tech A1,A6,A7,A8 & MVAC 609 + EPA 608

Unless stated otherwise, any question I ask is about my greymarket 1985 380SEL.