I've changed the oil maybe 5 times in 25 years. Compressor's always been in a heated garage and not used hardly at all.
As an aside, the one time it did see continued use was when my then-new neighbor started building his home. There were 3-4 guys working off've a small portable air compressor, it was ridiculous. After watching them struggle trying to "share the air" for a day or two, I got sick of it, threw an air hose out the side of the garage window and told them to make use of it. That made them happy....
Anyways, after a month or so of use, and after they'd finished with my compressor, I noticed that the compressor exhibited a slight mechanical noise, almost like a knock. Not necessarily bad, just not the noise it usually made. I changed the oil, which looked fine, and added some Slick 50 into the crankcase for good measure.
Believe it or not, compressor went back to its old self and has been fine ever since. That was 13 yrs ago and the compressor gets a work-out only whenever I use my sandblaster, about 3-4 times a year.
Compressor is a wanna-be 5hp from Costco and sorry to the OP for wandering off-topic