Originally Posted by TXiceman
The oil travels around the system mixed with the refrigerant. Most of the oil in in the compressor, but you can leak a small amount of oil at any point in the system. Too much oil is as had as too little oil.
Most cans of refrigerant do not contain oil. The ones that do are specifically labeled as having oil. To properly recharge the oil in the system, you have to remove the compressor and drain all of the oil out, then blow all of the lines to clear them of oil. Once the system is cleared of oil, you add a measured amount of oil to the compressor and close the system. Refrigeration oil is highly hygroscopic (sucks up water vapor like a dry sponge). The PAE and POE oils used with R-134a do not react well with water and can cause problems. So you will need to pull a vacuum to dehydrate the system to remove the water vapor.
The cans with oil in the refrigerant need to be charge as liquid (can upside down) to get the oil. Charging from the top of the can will pull off only the gas and leave nearly all of the oil in the can. I do not recommend liquid charging for the driveway mechanic.
For the older roof top units operating on R-22, they may have a dino based oil, or they may have a synthetic. It is generally not good to mix the oils on the refrigeration system due to the anti-foaming agent added to the oils. You can get by mixing the oils, but you never know the results until much later. The oils used on R-22 are also hygroscopic.
A refrigerant leak will usually show up as an oily spot at the leak (depending on the size of the leak). A little oil goes a LONG way.
There are lots of "short-cut" methods used by shade-tree mechanics that may work for a short time, but they will come back to bite you in the you-know-what every time.
If you want to fix it, fix it right
Again, Agree... But for the quickie fix that only needs to work for a short time (can be years, depending), cans and pierce valves are great. As Gary says, to go through all that would be cost prohibitive and you might as well replace.