No sludge, oil was dark brown when poured into the recycle jugs, not black but pretty dark brown against the black oil pans. I use AC delco air filters. I run my aluminum O-ring probe up and down the pleats probing for tears and no sludge came back on it, just oil. But I usually only go 6-7.5K just got busy and let these three cars slip by. No sludge in the bottom of the can either and I did not really drain them before cutting. Just a note, all three PL1s I just cut had oil that looked about the same. Some I wiped off better but all were dark brown. Also this car had a lean code in and the injectors had ramped up full rich so not sure it that would make it darker or not. That is resolved now but had been going on for about 3 months. What looks like sludge is just oil, they were dripping wet when I took the pics before I probed them LOL...