I have a couple questions.
For a 2000 F150 4.6 V8, I wanted to confirm if the rear differential fluid is 75W140 ?
I had it changed a coupe years back with Motorcraft brand, and I'm going to see if the level went down. I added a small bottle of friction modifier (4 oz.)

If I need to top up the rear differential, is it okay to use Royal Purple 75W140 with friction modifier ?
or should I top up with the same Motorcraft brand ? can the two be mixed ?
is it ok to add more friction modifier ?
I only put on 8000 miles since the rear diff was changed so the fluid should be okay...
I'm more concerned with the level going down, is it okay you think ?

thanks

 

If the level went down you have a leak that should be fixed. If its within 3/8" of the fill plug its full.

 

I don't see leaks in the driveway.

Is it okay to mix brands like Royal Purple with Motorcraft if needed ?

 

Yes you can mix them if needed.

 

Thanks

I asked the dealer about friction modifier.
I was told I don't need it.
I added it last diff fluid change for what its worth.
Royal Purple has friction modifier in there already.

 

If you had it changed two years ago then it might be time to change it again. You can pick up a gasket for your diff for a little under $5 like I did that way you don't have to wait for the RTV to cure.

 

Correct!
It was changed 2 years ago but the truck has about 8000 miles with this current differential fluid.
I checked today, its still clean.
I might change it....

Anyway this truck was not really driven all the years I had it. I used it only during the summer.
The first time I changed the diff fluids, the vehicle had 78 000 miles and it was 18 yrs old at that point.
I bought it used with 39 000 miles in 2005. I did the diff fluids in 2018.

Engine oils, coolant, trans fluid all changed generously.

The diff fluid was kinda never thought of.

In total the truck has 86 000 miles on it and it's a 2000 model.

 

Ah ok, then I would only top it off and check to see if there's a leak. My vent tube was what appeared to have caused what I thought was a pinion seal leak on mine but when the replacement was also leaking, I was told to check the differential vent tube as it gets clogged easily forcing oil out the easiest path it can. Might now be a bad idea while you're refilling your diff

 

Truck was parked in the driveway. There's a natural slope.
I topped up the rear differential with almost 1 quart of Royal Purple !
There's only a few squirts left in the bottle.

Wow !

I had this changed a couple yrs back at a shop. I dropped the vehicle off so I wasn't present to see how much was actually put in.
There's no leaks whatsoever. Is it normal for the fluid to be consumed or evaporate as time passes ?

I'll have to check the front diff now. I'm worried !

 

You shouldn't top up on a slope. Readings should be taken on level surface. Diff fluid isn't consumed. it's leaked. From the owners manual; "5 Your vehicle’s rear axle is filled with a synthetic rear axle lubricant and is considered lubricated for life. These lubricants do not need to be checked or changed unless a leak is suspected, service is required or the axle assembly has been submerged in water. The axle lubricant should be changed any time the rear axle has been submerged in water."
I can understand changing it once for initial wear in, but after that check for level and leaks only needed unless something broke or wore out, like clutch packs, inside. 4 oz of friction modifier should be all you need.

 

Does the front differential need friction modifier as well ?
4 oz of friction modifier is enough for the rear differential ?

Leaving the differential fluid in for the life of the vehicle is a frightening idea.
When I changed it at 80 000 miles, the mechanic said it was pretty dirty.

as far as leaks go, I don't see any....
Where does a rear differential leak from ?
I haven't seen any leaks. Does this happen while a vehicle is in motion ?

 

Unless your axles code has two numbers like say H9 axle (H92 this is a code for the front axle to have LS clutches in the pre 97 trucks code.) code for an LS 3.55 gears both front and rear on a 4x4. I can't remember how the 97-2003 4x4 code reads now, but for some reason, I want to say that it has to be either HH9 or H9B for the front to be LS as well.

4 oz of friction modifier is okay. I use two bottles for mine as they are old with a lot of miles on them. They can be a pain in the *** to change at times with pulling the rear- end apart. Soaking them for 24-30 hours in friction modifier so they are well wet before putting them back in. The tool to put them back in with Ford really likes it for the price or you make your own to reset them.

Leaking can come from Axles seals, chunk inspection plate, Speed sensor on top of the chunk, Pinion seal behind the nut, and the air vent on the top left-hand side of the chunk on the axle tube. (without there being a hole in the axle tube) You can set your rear tire on a hump sometimes and the axle moves just right for the seal to leak oil out. I have seen mine do that and get level and it never leaks again.

 

Interesting

 

When I changed my pinion seal and gear oil, I too filled it up on a slight incline and it took all three quarts of the Lucas gear oil I bought but is only supposed to take 2.7 quarts. As the above poster mentioned, it can leak from a lot of different spots. Mine was getting slung onto my gas tank and underside of my bed.

 

Ya, the ford dealer says the rear diff capacity is 2.5 Quarts.
There's better methods to increase fluid capacity of the rear diff. A bigger Diff cover will allow more gear oil capacity and supposedly better flow onto the axles.
I like B & M Hi-Tek & Banks