I have a ford f150 as well as my Dak and has synthetic oil in it for the diff and the owners manual says fill for life as well and I could not get a answer from ford either, but I have a friend who is a mechanic and he says as long as you are running synthetic and not going through deep water where it may leak into the diffs or t-case you are good to go. Thanks for your info Sawfish. Based on the price they want to charge me to do the 4x4 service I think its just a money maker for them, Like you I have all the stuff in my garage to do it myself for way less.