Some industries such as food industries and pharmacetical industries and hospitals have to use oiless compressors.Just like many other things, air compressors come in a wide range of quality and prices.
Those old Jiggs and Maggie cartoons of many years ago in the comic books and newspaper comic strips,,, (I remember them well) back in the 1950s Ford used to have some great big Jiggs posters on the walls or hanging around in the Ford factories of Jiggs saying, SURE, AND QUALITY COUNTS!
I am sold on oiless air compressors.Have you ever tried to spray paint something and oil is in the air line? I can tell you it doesn't work!
I think 30 weight motor oil for the kinds of air compressors that are lubricated with oil is as good as anything else.That is probally what the gas stations/service stations/auto repair shops many years ago used anyway.The oil cap on the Briggs & Stratton engine on little walk behind lawn mower says to use 30 weight oil. cuhulin