Have a '97 with the NV4500 trans. Changed the trans oil to Ansoil 75W-90 @ 943 miles and added an Eaton PTO filter. Trans did not shift quite as easily as with the Syntorq so changed oil to Ansoil series 2000 75W-90 @ 13,670. Oil was clean; as was filter. Shifted much easier after the change. Changed the oil today at 68,500. Oil looked like dirty engine oil; filter was black and bottom of trans housing was covered with grey-black sludge. No flakes or chips just sludge. I've never seen trans oil this dirty in 30 years. Have had several old Power Wagons with crash box transmissions which never got this dirty. I've never had the truck off road; tow infrequently. 90% of the miles are highway miles. Any body have any comments or reasons why the oil is so dirty????