Dont know what it was black hard as hell crap.We were thinking compressor windings so we changed out compressor had about 4 inchs of liq line plugged. I felt like a jackass for not seeing the problem. On this unit it has a sensor right before the filter dryer wrapped with insulation when i was checking it out i seen filter freezing so i changed the filter if i would have removed the insulation i would have seen it was freezing about 3 inches before filter right were it was plugged other tech found it in about 30 minutes.
I had some of that stuff in a 2 ton Trane condensor once, don't know what the heck it is either. That was a couple years ago. Just replaced the condensor coil, kept the compressor. Still running after 2 years.