If it "works well", I'd leave it alone.Works well, just needs some gas.
If it works "just OK", you may just have lost a small amount of refrigerant over time, and addition of some R134a would be justified. About 5 years ago, my untouched AC system in my '98 Frontier needed about 6 oz. R134a at about 190K miles, Arizona heat. That sufficed until last week when the high pressure hose developed a pinhole and I had to replace that line.Works well, just needs some gas. I want to put a small can of oil just to be safe.
Don't see any leaks. Looks to not have been worked on at all. Guy I got it from said me never charged it.
That's where I first noticed the pinhole, in the rubber hose, at 219.5K miles. My replacement line/hose leaked at the connection to the compressor, around the O-ring, described in my thread http://www.clubfrontier.org/forums/f46/1998-frontier-thread-179905/index5.html page 4 and page 5. This might have been accelerated by my fan clutch being bad causing higher pressures than optimal and I didn't notice as AC was still cold, and engine coolant hadn't gone up from its normal spot.The high pressure hose attached to the compressor is a pretty common point of failure on these trucks. Look for signs of oil around the crimps if you are slowly losing refrigerant.