Thanks so much to everyone who contributed to this subject, especially the_natrix. We have a 2008 MKZ, our PTU made it to about 80k miles before it started venting lube. Once I saw the mess on driveway, immediately raised car to investigate, found lube from front to back on underside of floorboard. Researched it (here mostly) and decided to pull out of car and disassemble. Found same sludge build up inside, washed everything out with solvent. I added a 3/8 drain plug in bottom of case. In my opinion way to little lube for type of gearbox and load, I plan to change fluid once a year. Also I replaced all seals except for pinion to rear differential output, was worried about messing up crush washer for gear mesh. So far no leaks and no whining, seems to be working well.
