Ford Truck Enthusiasts Forums

Ford Truck Enthusiasts Forums (
-   1997 - 2003 F150 (
-   -   Rear End Oil Help (

Jake86us 08-08-2014 11:21 AM

Rear End Oil Help
Hey guys, i'm new to the forum and just wanted to ask you guys a question. I have a 2003 F150 king Ranch and its time to change the rear end oil, what type of oil can I put in it and what would be the best for pulling trailers and what not. It has an axle code of 26, so I know it has a nonlimited slip 3.73 rear end in it.

thanks for any help!!

70f100longbed 08-08-2014 12:59 PM

75w140 synthetic oil is what Ford recommends

Jake86us 08-08-2014 04:44 PM

I looked closer on the axle and there was a half-painted sticker that says 75w90 fehp only and on the tag on one of the bolts it says the same thing, but in the manual it says 75w140 syn oil. Which is right? I looked up the axle code and since its not a limited slip, can I use any brand syn gear oil? Valvoline or royal purple or amsoil. Which is best?

steve(ill) 08-08-2014 05:56 PM

the 75w 90 is an old spec... ALL FORD trucks are upgraded to the 75W140 synthetic... Don't have to be a name brand, you can take your pick.

Jake86us 08-08-2014 05:58 PM

oh ok thanks for the reply!!!

blupupher 08-08-2014 06:16 PM

Just go to Wal Mart and get 3 qts 75w140 synthetic Wal Mart brand gear oil. It is the least expensive and will work just fine for you. No need to spend 2x as much for Mobil 1 or Royal Purple. I used that I got on Amazon (I had a gift card, but even full price it is just a bit more than the walmart brand).
You can also pick up a tube of gasket maker while there to reseal it (I used Ultra Black on mine and no problems).

Scalemandave 08-08-2014 07:23 PM

Don't Forget the Limited Slip Additive if its a Posi

steve(ill) 08-08-2014 08:36 PM

Local Wal Mart here only stocks 75 w 90 synthetic. Have to go to Auto Zone and pay 50% more..... Still look for the economical brand.

blupupher 08-08-2014 10:40 PM


Originally Posted by Scalemandave (Post 14568810)
Don't Forget the Limited Slip Additive if its a Posi

good point, but the OP said they did not have a LS.


Originally Posted by steve(ill) (Post 14568988)
Local Wal Mart here only stocks 75 w 90 synthetic. Have to go to Auto Zone and pay 50% more..... Still look for the economical brand.

The just order some Lucas from the link I posted above from Amazon. $11.99/qt and free shipping on orders over $35 (and 3 qts is what you need, so you will get free shipping).

FordTruckChuck 08-08-2014 11:23 PM

Should I have changed it already in my 98 with 81k miles? I thought it never needed to be changed.

blupupher 08-08-2014 11:30 PM

While it is a "lifetime fluid", your looking at 16+ years on this fluid with your '98. For $40 or so you can add new fluid that will last for as long as you own the truck.
Also, have you even checked the fluid level in the diff? Could be low from a small leak (or underfilled from the factory?).

Could you not change the fluid and be fine, more than likely yes, but again, it won't hurt and will give you peace of mind.

Now if you tow a boat or off road and get the rear end in deep water (mid axle or higher) or tow/haul heavy loads a lot you probably should change it every few years (or more often if you do it a lot) since water and gear oil (even synthetic) don't get along.

FordTruckChuck 08-08-2014 11:43 PM

Thanks for that, Blupupher.
It has never towed anything. Unfortunately, it has been towed more then a few times but that is another matter.
What is the procedure for replacing it?

blupupher 08-09-2014 12:08 AM

There are a few how to's around the net for doing it, not real hard.

Basically you need a drain pan for the old fluid, w 3/8 breaker bar (to remove the fill plug, it is in there tight), I think a 12 mm socket for the bolts on the diff, gear fluid (and Friction modifier if you have a limited slip) and some gasket maker (I used Permatex Ultra Black, but there are other options).

Some drain it warm, others cold. I did mine cold.

Put the pan under the rear axle, loosen all the bolts on the diff cover 1/4 a turn. once all loose, start removing them (bottom to top) leaving the top 3-4 in. If it has not started draining, take a small screwdriver and pry it apart and the fluid will spill out.
one note, gear oil stinks horrible, so wear nitril gloves if you have them.
Once it is all drained, take the rest of the bolts off and remove the cover. clean off the inside of the cover (I used brake clean to get the oil residue off and a plastic scraper for the gasket maker stuff). get all the sealer off the diff housing itself also. Now get a few rags (or a gloved hand) and "scoop" out the oil in the bottom of the diff, there is about a cup of oil down there that you want to get out.

Now clean the diff sealing area again and wipe it off clean. Apply the gasket maker (several options out there, just follow the directions on it).
With the ultra black, I applied a 1/8" bead all the way around the diff cover, with a little bead around each bolt hole also. I let it skin over (10 minutes or so) then installed the cover finger tight. Let it sit for about 20 minutes then tightened all the bolts in a criss cross pattern (did not use a torque wrench, just did it GnT (Good and Tight)).

Removed bolt from fill hole using 3/8 breaker bar and then fill the diff with gear oil (2.75 quarts I believe). Basically fill it till it runs out the hole. If you have a limited slip, add the friction modifier to the 1st bottle after it is 1/2 in, much easier to do that way (and the FM sticks much, much, much worse than gear oil). once full, put the plug back in, clean up and your done. Take it for a test drive and look for any leaks.

I may have missed something but this is the basics of it.

edit: Here is a how to with pics, say basically what I just typed out with a few small differences, but nothing major.

steve(ill) 08-09-2014 07:50 PM

When you drain you will notice some "crap" will come out with the oil. This is from the original casting, and some bearing dust.......... ALWAYS helps to drain the factory oil out after a period of time and put new oil in.. Fresh oil is better on the bearings than having "crap" go thru the bearing.

FordTruckChuck 08-09-2014 10:41 PM

Thanks for all that, blupupher. Sounds like something I'll be able to do. I think that I will make sure I'm able to loosen the fill plug prior to draining it.
Good point, steve. I should have known better to not have replaced it yet.

All times are GMT -5. The time now is 09:16 PM.