BITOG.... I've come to make the proverbial mountain out of a molehill in regards to a gear oil choice. I have a 2004 F150 that I recently purchased with 125k miles. It has a 5.4 liter engine and is 4x4. It's in great shape in and out, and runs well. The manufacturer says the axle is "lubed for life", which I'm sure incites some hearty debate. The trans fluid looks/smells fresh, so it's been serviced recently. The axles however show no signs of ever being serviced (no tool marks on the fill plug, cover looks to have never been off). So I'm interested in replacing the fluid in the front and rear axles. Usage- weekend trips close to home, possibly some medium towing (5k pounds) infrequently. In doing some background research on gear oil, the owners manual specs a 75w90 synthetic "fuel efficient high performance" gear oil. I cannot find it anywhere, and along the way found some TSB information online that years ago Ford advised its dealers that the aforementioned oil should not be used, and instead a 75w140 synthetic should be used (and even went so far as to say the tag on the rear end specing "XY-75W90–QFEHP" should be scratched through). I have narrowed it down to 2 (open to more suggestions though), and would love to hear your feedback: 1) Motorcraft 75w140 synthetic (XY-75W140-QL). Pros- I find some value in using OEM fluid in the vehicle, and have been impressed with what Ford/Motorcraft has done with their oils and filters. If they have enough confidence to say "lubed for life" I would imagine it's a great oil. Cons- About $5 more expensive per quart than the other choice below, plus it appears I'll also have to buy some friction modifier. 2) Mobil 1 75w140 synthetic. Pros- have used mobil 1 in the past with good success, feedback from others using it are always very positive, less expensive than the option above, and already includes friction modifiers for limited slip. Cons- not OEM fluid. Thoughts?