I started to notice that my car consumed more oil over that past 6 months. Over the past year I would add about half a quart of oil every 3000-4000 miles. Recently I had to add almost a full quart after less than 3000 miles. There were no visible obvious oil leaks except for the residue on the underside of the turbo intake housing. There was also residue around the oil fill cap (on top of engine). I suspected that both were caused by backed up pressure due to clogging of the breather (positive crankcase ventilation) system. The back pressure likely pushed the seal in my turbo open and started feeding oil into the air intake system where it eventually got burned up. So the next project will be to rebuild my turbo. At 145k miles, I was overdue for the PCV job anyways (usually done at 100k). I use Mobil1 oil only but even with this I saw buildup.