Ok I am setting aside an afternoon to change my diff. fluid and front axle fluids. Rear Royal Purple with modifier (75w140 or 85w140) I believe the manual says 80w140 but I cannot find RP with this weight. Same with the front axle (75w90, 80w90 or 85w90). I do not do much 4X4 or towing. What do the experts think or recommend. No mods except from my tag line.

 

You are doing good!

Diff fluid is one of those things that if you don't ever tow or get it wet, it last forever.

 

I used this stuff on my rear diff:
Mobil 1 Synthetic Gear Lube LS 75W-140

I found their friction modifier worked fine right out of the bottle.

My front doesn't need service yet, but I will use this when it does:
Mobil 1 Synthetic Gear Lube LS 75W-90

I am certain the RP is just as good.

Here is my thread from when I did the rear:
https://www.ford-trucks.com/forums/8...with-pics.html

 

Bill,
THanks again for posting all the information and pics. Its VERY helpful to us inexperienced folks. Sorry for the tangent here, but, have you serviced the transfer case on your truck? I am trying to find out exacty what type and brand of fluid is the best for this. I've read where others are just saying to put ATF but not to use Mercon SP... I plan on servicing my front/rear diffs and transfer case at the same time. You've nailed down the procedure for those, i just need to find about the fluid for the transfer case ( I don't think it has a very large capacity, don't mind spending a little $$$).

Also, i am trying to find a part # for the stock front diff cover. I would like to change both front & rear covers when I do the service. I just ordered the rear diff cover just like what you used. Do you know where i can find the front?

Thanks again for all the help. Sorry for the slight thread hijack..

2005, FX4, 127k miles, studs, 4" exhaust, SCT tunes, coolant filter, Edge insight, ISSPRO FP

 

I have RP 75W-140 in the rear and 75W-90 in front diff. I also have Mag-Hytech diff covers that doubles the oil capacity of both diffs. I tow my 6,000 lbs. boat and my 12,000 lbs. dump trailer, but not a lot. I changed the diff covers just as a piece of mind/reliability thing and should never have to change fluid again.

You don't need the limited slip modifier with RP synthetic.

DSMMH

 

So 75w-90 is OK for the front, and 75w-140 in the rear. As long as I use RP I don't need limited slip modifier? Is it already premixed in the gear oil?
Thanks

 

Yes, I did my T-case about 2 months ago. It could not be simpler. You just pull the lower plug and drain, then fill through the upper plug until the level is even with the plug hole. You will need a big metric allen wrench for the plugs. Don't recall the size at the moment, but Sears had it. (on edit: 10mm Allen)

I used Mobil-1 Synthetic ATF.
Mobil 1 Synthetic Automatic Transmission Fluid (ATF)

It takes a little over 2 quarts, so buy 3 quarts. I was able to do mine without removing my skid plate by bending up a piece of aluminum flashing as a makeshift funnel.

I would not use SP, and in fact some of the bottles say not to use it in a T-case.

I don't know the part number for the front Diff cover, but PartsGuyEd would be able to find it for you quickly.

 

Bill,
Thanks again for the info. Sounds great, changing the CK fluid is straight forward. The only part about the CK fluid that was concerning me is, using ATF. When others were saying to use ATF, i assumed there was a particular grade (i.e. 15w40). Guess not, ATF is...ATF. Sounds great, i'll probably use the Mobile 1 synthetic, sounds like its popular. Also, I'll check with Ed "the parts guy" on the front diff cover.

Thanks

 

75W-140 in the rear and 75W-90 in the front is what the owners manual and the maintenance manual recommend.

RP adds hypoid friction modifers to all gear oils.

Royal Purple Gear Oil

I use RP diff oils in my X diffs, ZO6 diff, Scarab Bravo 1 outdrive, boat and dump trailer oil wheel bearings. Before I changed to RP in my ZO6, I was experiencing slight front end push when cold and turning.

I would highly recommend you change the transfer case oil to RP Synchromax. I think it takes about 3 to 4 quarts.

Royal Purple Manual Transmission Fluid

DSMMH

 

75W-140 in the rear and 75W-90 in the front is what the owners manual and the maintenance manual recommend.

RP adds hypoid friction modifers to all gear oils.

Royal Purple Gear Oil

I use RP diff oils in my X diffs, ZO6 diff, Scarab Bravo 1 outdrive, boat and dump trailer oil wheel bearings. Before I changed to RP in my ZO6, I was experiencing slight front end push when cold and turning.

I would highly recommend you change the transfer case oil to RP Synchromax. I think it takes about 3 to 4 quarts.

Royal Purple Manual Transmission Fluid

DSMMH

 

i just changed out my front and rear differentials a week ago.

the local stores didnt have the brands i wanted, i got stuck with...

MasterPro GL-5 80W90 for the front differential, ( cheap crap probably )

Valvoline Full Synthetic "limited slip" 75W-140 for the rear ( about $12 per bottle )


now i am nervous... do i need to add the friction modifier ? i always thought that had to go in NEW gears... so if i need to add it, do i put it in the rear only ? or both ? how much per differential ?

 

I'm sure what you used is fine. You used a full synthetic in the rear, and that is critical, and because it was "limited slip" it has the friction modifier built in. Just like Royal Purple does, and just like Mobil-1 does. Some of them you can find with or without the friction modifier, but if your bottle said it was for limited slip then it has it included already. You don't need to worry about the FM unless you are getting grabbing and chattering when turning corners.

Royal Purple is okay, but don't make a religion out of it. Valvoline is fine too. The Masterpro sounds like a house brand, but since it sees limited duty in the front, I would not be concerned.

 

Friction modifier is for clutch plates, not gears which is why non-LS and non-posi units don't need it.

 

One more question,
Is there any problem with adding the friction modifier along with a full synthetic oil that already has FM in it (mobile 1 or RP)? Or is it just an overkill? Just want to double check with you guys, don't want to harm anything having too much FM in the gear oil. It may be just a waste of $$ adding the FM, but I would like to stretch this interval (as long as i can keep it out of the water). To all, thanks for the input and quick responses. I appreciate it.

Nevermind, i just answered my own question. Per Bill's link above for the mobile oil, "..For axles requiring the highest level of limited-slip performance, OEM specific LS additives can be added to this fluid."

 

While all you 'diff' guys are out there: just bought and 05 auto 4x4 with 55K.
Should I change out my 2 diffs and transfer case because I have know idea what this truck has been put through?
It doesn't look like anything has been changed to date (very dirty and rusty).
Did the tranny & all filters but hesitating on overkill on the rest.
I've owned trucks that were 20 years old with all original diff fluids so is it really a worry on a 5 year old truck?