Ok I'm about to change the rear diff plate on my 1999 ford f150 xl truck. It's 4x4 offroad and has a 5.4lt Trinton V8 in it. Now my question is, what can I use instead if any, for the diff fluid? The 75w140 , which the book said to use, is $23.00 a liter and the 75w90(syn) is $10.00 a liter. Also do I need to add a additive to the fluid whien I change it?


Thanks

 

The 75w140 is recommended because it is thicker and will hold up to the heat better. Use synthetic and if you have a limited slip, make sure to get the posi additive too. Also, the thicker fluid is less likely to leak.
Jimmy

 

Use the heavy stuff an do it right!

BUT, don't get it from ford its a rip off. You can order the right stuff from Jegs or Summit racing among others. It might also have the additive in it if I remember correctly. It only costs $7 a quart, vs the 33/qt Ford tries to get for it.

Chris

 

You only need the thick stuff if you do a lot of towing.

 

I don't do any towing and very little 4X4 offroading. It looks like 50% of people say use 75w90 with the additive and 50% say use the 75w140.
I can get 85w140 here locally for a good price but not the 75w140. That was why I asked if I could use 75w90.

Thanks

 

Use what you want, but remember that ford says to use 75/140 synthetic only and I'm sure there is a reason for that.

 

Since Ford went to the 75W140 we see a lot of diff bearing problems. I'm not sure if it is due to the oil or to them possibly changing bearing suppliers or something like that.
A lot of customers dont want the expensive oil put in so we use plain old 80W90 at their request and I cant recall that I've seen any problems afterwards.
Still is probably safest to use what Ford wants though.

 

i only use 75w140 full synthetic. Around houston you can buy it at Oreily's auto parts. About $28 for half gallon which is plenty enough to do the job. Brand is sta-lube is i remember correctly. Since you have the 4x4 you will also need buy small tube of gear additive. details in your owner manual.

 

Name Gear Oil SAE 85W-140
Valvoline Blended...
3 - 4x4's (2 ford's)with a total of 230,000 miles, front and rear diffs never had a problem and lots of hard towing in one and lots of hard off-road in the other. (third is a jeep)

 

Is that 85w140 synthetic and did you add the additive to it also?

 

It's their blended..."DuraBlend" is what they call it. And I still use the friction modifier also. I say no problems with all those miles, but on the 99' F150 4x4 with 9.75 rear, I grenaded the Trac-Loc once, but that was pretty much my fault (gotta play hard sometimes) and the 119,000 miles. Rebuilt the Trac-Loc and it's goin on 130,000. The Expedition has never had any problems, even with the wife drivin, and the Jeep diff's have never had a problem with over 90,000 VERY HARD mostly off-road miles. 3 vehicles-all 4x4, 230,000 hard miles...gotta be good stuff.

 

By the way, you may not have have Limited Slip rear end, and thus may not need the friction modifier. Check the axle code or crawl underneath and look on the pumpkin tag. Just because it is 4x4 doesn't mean it has the Limited Splip by default. There are threads with axled codes on this site and the tag will simply have an "L" with your gear range listed (355L or 373L) is it is Limited Slip.

 

Ok I got the numbers off the tag on my diff plate. It is as follows:

S 918A
55 9 75 9B26

I also took a pic of my door tag but can't show it here.

Can you tell from that info what I have? Is it LS or no?

Thanks for the help.

Greg

 

Bettin there was a "3." in front of that "55". Next to bolt, can't always see it, but there ain't no "L". So it must not be the limited slip. The "9 75" is the axle code, better than the "8.8". Do a search on this site for "axle codes", it's here somewhere. This will give you the breakdown for your axle code just to make sure. Besides lookin around this place is a wealth of information.

 

Compliments of one of the moderators:

Axle Codes (LS=Limited Slip / Traction Lok):
F-150:
19 – 3.55 Non-LS
18 – 3.08 Non-LS
H9 – 3.55 LS
B6 – 3.73 LS