My 2000 F150 let me know it needed changing by starting to pop and crackle when I turned at about 80k miles. My 02 bought with 69k miles on it didn't make any noise but when I had to replace an axle and bearing, it got changed at 133k. Then I changed it again when I pulled the other axle again to see if it was causing my bearing roar that I haven't diagnosed yet. I used Valvoline 75w140 synthetic both times on the 2002. The first time I added a bottle of Ford viscosity improver. The second time I realized that the Valvoline stuff already had it added and didn't add the ford stuff. Afterwards both times the traction lock worked/works great.