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?
Limited slip rear end, F150 gear oil.
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Jordan_FX4
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I changed my gear oil and used the recommended 75W-140 in my F-150 with the 3.73 limited slip and it's done fine so far with no chatter or binding. Changed it at 118,xxx and now at 143,xxx. I used Amsoil Severe Gear 75W-140 and added their friction modifier just to be safe.
As far as fuel economy I think the added gain would be negligible to nonexistent. Personally, I don't see any reason to go against Ford's recommendation as it seems to be doing just fine in their trucks and I figure they had their reasons for spec'ing 75W-140 but that's just an opinion. Hopefully this helps.
As far as fuel economy I think the added gain would be negligible to nonexistent. Personally, I don't see any reason to go against Ford's recommendation as it seems to be doing just fine in their trucks and I figure they had their reasons for spec'ing 75W-140 but that's just an opinion. Hopefully this helps.
when i had an f150 i used the motorcraft 75-140 and added fords friction modifier. if you use another brand such as M1, RP, Amsoil or others that contain lsd additive you should be ok, however if the rear end isnt as quiet as it should be on the others you may have to buy the friction modifier from Ford and add a little at a time until it is quiet. i would not add the whole 4oz at first if the fluid your using has lsd additive already. the idea is to get enough but not too much additive. that's my opinion. good luck!
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This is a post for Jim Allen. He has done a lot of research and testing of different viscosity gear oils.
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.
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.
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