My IR T-30 compressor has the residue of 40 years in the bottom of the crankcase. It seems to be all liquid, but thick enough so that it doesn't flow very well, and even with the compressor blocked up so the sump is tipped toward the drain there's still a fair amount that won't come out.
Changing the oil several times would likely clear it all out, but there's enough water in the goo that it does make the new oil creamy pretty quickly. What about flushing with ATF or other light oil? I'm not talking about running the compressor this way - just pour in a quart, slosh with a long handled brush or something thru the filler plug, drain, repeat as required.
The advantage I see to using ATF is it's *much* cheaper and much less viscous at room temp than compressor oil. If it was a gearbox I'd flush with kerosene, but don't want to introduce anything that volatile/flammable into a compressor. Any thoughts on this?