After reading your last post and similar posts from others..I decided to look into my rear differential. You were right...I don't see how this stuff can be considered "good for life". The plug was badly caked with sludge and the factory fluid was a dark grey. I changed mine out with AMSOIL this afternoon.
Thanks for bringing this to my attention!
2000 F-150 XLT, 4x2, 5.4L Supercab, Superchip, Styleside, Black with silver two-tone, 3.55 rear, class III towing package with super engine cooling/auxiliary transmission cooling HD package, Heavy duty shocks, 4 wheel disk ABS, overhead console, sliding rear window, keyless entry, dark graphite interior, in dash CD, factory leather wrapped steering wheel, FORD bedliner, FORD black tubular cab steps, K&N air filter, mar-hyde rubberized undercoating, Modine climate filtration system, Bugflector II, Mobil 1 oil, build 7/2000