I have a Harbor Freight 8 gallon 120PSI compressor that has paid for
itself half a dozen times over just in fees saved for blowing out my
sprinkler system.

I use it intensely once a year for sprinkler blowout (9 zones x 6
repeats per zone) and about 1-2 times per month for light project use
(usually just fills the tank a couple of times).

The compressor takes about 25oz of 30 weight non-detergent oil.
The oil level seems to remain pretty steady in the window.

- Should the oil be replaced at regular intervals?

- If so, is there a good general rule of thumb for how freqently to
replace (either in terms of calendar time or usage)?

- Also the manual says use "30 weight non-detergent oil". Can I use the
oil so labeled at an auto parts store or do I still need to buy "special"
compressor oil of that weight?

Thanks