I worked on a medium temp. R-22 rack today. Compressor was tripping on oil failure. I valved off the compressor and drained the oil out to check the screen. This screen was covered with a thick silver paste. I cleaned it real good and put it back in and all was fine. The oil has been completly changed on this rack 2 times in the past 2 months and the oil is still filthy. How many times does it usually have to be changed before it is clean? And what is that silver sludge anyways?