Have you actually looked at an oilless compressor, or are you just
guessing? Where is the bronze bearing you are talking about?
I have an oilless Sears compressor. The motor and crank bearings seem
to be sealed ball bearings, so no oil is needed there (unless you want
to count the oil that is part of the grease that is packed into the
bearings). The piston rings are apparently some sort of graphite that
slides dry on the cylinder liner. Thus, the air is not exposed to oil
at all - which is the real point of an "oilless" compressor. No bronze
bearings in sight.
Oil-lubricated compressors have a crankcase, and use oil lubricated
bearings and the piston is oil lubricated. This apparently gives lower
piston wear and longer time between rebuilds, but you end up with a bit
of oil in the compressed air.
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