I looked last night and saw no unusual oily residue on anything. I also did read that if the A/C isn't used much (especially in older vehicles that sit for long periods of time), that the compressor shaft seal can become dry and refrigerant can leak from there. Having said that, the 92 year old man I bought it from hardly ever drove it. When I went to look at it before i bought he hadn't drove it in months. It was just in his garage sitting there lookin' pretty LOL. I have no clue if that has any merit to my low refrigerant level, but I am going to keep an eye on it and use the AC often to see if that compressor shaft seal becomes lubricated from the oil in the AC. And if it continues to leak, it will go in the shop or I will get the proper equipment to find/fix it.

 

Yes.
All automotive compressors are designed to leak a small amount of refrigerant/oil through that seal to keep the seal lubricated. In your case, it could be possible that all the refrigerant just leaked out over time. So you might be lucky with simply adding R134a, and be OK for a few years. So in your instance, I say simply add more R13a, and I'd shoot for about 250 psi on the high side gauge at about 1800 rpm. Low side should read 25 to 40 psi then.

I have a 1998 Frontier with 250K on it (mine since 2004) and original factory compressor, and I only needed to add refrigerant once I got a pinhole in the high pressure rubber line on a 119F day, and the refrigerant all leaked out. But I also have a 2004 Frontier and a couple of years ago I needed to add 6 oz. R134a, so that leaked enough to affect performance in about 13 years.

My own 1988 Mazda B2200 leaked out most of its R-12 in the last 6 months, I'm currently leak checking this. My own cause could be related to a bad rubber seal in the AC service valve cap, see photo.

 

I will check my gaskets on my service valve caps as well. I will say this. It sure blows icy cold now. I will also mix some car wash soap and water and pour that on the seals as well to check for bubbles and any leaks anywhere.

 

Great ! Don't add or remove any refrigerant. If the cooling out the vents is fine, then that's the "performance check".

Even if you need to add another can or half-can in summer, I'd say might not be worth tracking down such leak of that size, since you're using R134a which has been $4.88 a can at Walmart and Home Depot.