Philosophical rear differential question

2000 F-150 5.4L Harley Edition. looking to change rear diff oil. Manual says 75W-140 plus friction modifier. I was going to go with Royal Purple, supposed to have modifier already added. I get alot of looks when I tell guys it takes 140… Could I get away with 75W-90?? was wondering the drawbacks… maybe positives?? I’m getting mixed reactions.
now this friction modifier really stumps me… what can you add to oil to modify it?? I believe they think I need it but can’t see the concept… I just don’t get it… thanks for listening.

Usually, when a “friction modifier” is called for, it’s to accommodate the clutches in a limited slip differential. Without it, the rear-end can “chatter” when cornering and accelerating at the same time…Some grades of gear oil already contain this snake juice…The important thing is you use a GL-5 rated 75W-140 lubricant…75W-90 might not be heavy enough for severe service or very high temperatures…

While most Fords have a magnet in the fill plug, they are weak and not very effective…I like to install a powerful rare-earth magnet removed from a computer hard-drive on the inside of the rear cover which will remove even microscopic metal particles that gear-boxes can produce…Cheap insurance, especially should a wheel bearing fail, releasing an explosion of metal fragments into the axle housing…

Why fight with the requirements Ford puts in? You don’t change this very often, so it can’t be cost. Do you have any idea what a new differential costs?

There is no good reason to use anything but EXACTLY what Ford recommends. Of all the places a ‘designer’ oil might help, your diff rates last.

Don’t worry about funny looks; lots of people don’t know everything. Friction modifier is just another thing that says that chemistry is strange when it hasn’t been studied, you just have to take the specifications and go along with them. The stuff really exists, we think.

You could get away with conventional 75W90. If you were stuck in the desert and that’s all you had it would work. But your diff was designed to be lubricated with 75W140 and apparently you bought a limited slip diff which also requires some friction modifier to keep the clutches from binding/chattering on turns. So use the proper oil and additives. Oil is cheap. Differentials are expensive.

Don’t get too hung up on the Royal Purple thing. Apparently, that purple dye they put in their oil is some seriously expensive stuff. Personally, I like Valvoline and Mobil. Neither have ever done me wrong. Whatever you use, use the weight Ford calls for and be sure to add the friction modifier. If it were my truck, even if I spent the huge bucks on the Royal Purple (or, more likely, someone gave it to me), I would not install it in a limited slip differential without adding friction modifiers. Differentials are expensive, and limited slip differentials are even more expensive.

Made just for you…

great info. This is in my plans for warmer weather. thats a great ebay auction. have it saved.