Once a company was called to check the system and charge it. Because they had difficulty with one of the valves, and installed saddle valve. Unfortunately, it leaked. I purchased butane torch and brazed the hole closed. At that time, R-12 was sold for about $0.69 per pound and was available at most automotive supply stores. In 2004 R-12 was going for about $80 per pound. ($10,000 invested in R-12 around 1980 would be worth about $1,159,420 in 2004.)
Because the furnace was oil, a company was called when it would not start a couple of times. The first - the burner needed cleaning. The second - 2003, the ignitor contacts were dirty. I did not have much experience with oil burners previously. It just ran and ran and ran. It was a commercial unit.
Other maintenance items, replace a contactor after being struck by lightning. A starting capacitor for the older compressor. The fan motor both inside and out. A couple V fan belts outside. The inside belt never needed replacing.
Replace the pillow bearing on the blower inside.
The exhaust flue deteriorated and was replaced.
The contactor covers did not manage to keep roaches out. It was necessary to occasionally (about every 3 years) clean out the roach dung. That is a problem with electrical equipment located outside. If it is not absolutely bug tight, they seem to get in. In the old system, it was as much as 1/2 inch deep.
Rodents would crawl in the refrigerant pipe access hole and make nests. On a few occasions, they had made a nest inside the blower. When the unit started, the nest eventually flew out and the centrifugal force pinned them to the blades to their demise.