Originally Posted By: HM12460
Getting ready to change the rear differential fluid on my F-150. It has 101,000 miles. Our friends at Ford say to use a 75W-140 gear oil. It is a limited slip with 3.73 gears. I'd really rather use a 75W-90 full synthetic (Mobil 1) with the LSD additive already mixed in. Hopefully pick up a little better fuel economy. The truck does not do any heavy hauling or towing and as such, I don't see the need for 75W-140 gear oil. Thoughts?
In my 07 F150 with 3.73 LSD, I changed the oil last year to Amsoil Severe Gear 75W-140 at 20,000 original miles. I omitted the additive, as they recommend, but I had bought some anyway incase it chattered. Well it chattered awful, so I immediately added 2oz of Amsoil additive. Still chattering. Added 2 more ounces. Still chattering. Added 2 more. Still chattering. Added 2 more. Still a bit of chatter. I called Amsoil tech line and he put me on hold for 10min while he researched my problem. Came back and told me to add 2 more ounces, but no more than that. So I put in 2 more healthy ounces (almost 3), and now it is quiet, but I have nearly 9 ounces of Amsoil SlipLok in there! My conclusion: The Severe Gear fluid is real good stuff, but Amsoil's SlipLok is JUNK. Stick with Ford's Friction Modifier. Even GM guys swear by the Ford stuff. I am still not comfortable with the contents of the rear end, but I don't have time to change it out and use Ford FM.