I know the diff tag says snythetic but can i use 85 90 in the rear diff or does it have to be snythetic. Also if i cant, can i drive like 50 miles on it and not hurt anything. Thanks

 

You can drive 50 miles, but get the right fluid in it as soon as you can, which is 75W140 synthetic - and if it's a LS, you also need friction modifier.

 

what does LS mean?

 

Limited slip.. the type of rear axle you have.

 

i was told when i pulled the pan that its posi and even if its not, what do i need to get for friction modifier. I baught 85w140, is this good enough or no?

 

posi is the same as limited slip in your case.

85 might be a little heavy for the rear, don't really know enough to give you a definite answer for that, but i would go with the manufacturing specs that you were given.

for the friction modifier, your best bet is to go to ford and ask for friction modifier.

 

No, 85W-140 is not full synthetic. You must use 75W-140 full synthetic, any brand. Example: http://shop.advanceautoparts.com/web..._C|GRP2035____

Ford sells friction modifier at the parts counter, part number is C8AZ-19B546A
(EST-M2C118-A).

 

the bottle says i can use where any 75w 140 is used....******ing auto parts stores, people think they know it all. This is the 2nd time they have been wrong. This is getting a little pathetic...First they say "OHH ALL REAR ENDS USE 85W 90" now they say "every rear end uses 85w 140" Come on, give me a break. I have already spend $50 dollars and now i am done. Can i use the 85 140 and still add friction modifer down the road and be alright?

 

alright so i got the 75 w 140 can i drive on it for a 100 miles or so before i need the friction modifer or do i need it right away. If thats not a good idea, no problem ill just take tomorrow off . What is the mixture for the friction modifier.

 

You can drive around the town without the friction modifier, but don't take it out on the interstate for any extended amount of miles without adding it. You will burn up that rear. http://www.morethantires.com/catalog...92/1013489.htm

 

The friction modifier is needed to get the viscosity just right so the traction lok clutches will slip going around corners when you are not stomping on the gas. Without the modifier the clutches won't want to release when cornering and you will hear them popping and snapping when they do slip. It's not a problem to drive in a straight line without the modifier. It's only when you corner that you will know it is not right.

It doesn't make sense to me to use something other than what Ford recommended unless you know in advance it will work and I don't think any of us can tell you that. It may work. Who knows. I'm sure the gears won't be damaged by the 85-140. At worst the traction lok clutches won't release properly.

 

You put the whole 4 ounce bottle of modifier in. That should be done as soon as possible.

 

FRod,

What brand of oil did you get? There are some synthetic brands that claim that you do not need the friction modifies as it is incorporated into the oil. Redline is one of them. I have used it in the past without the modifier. I figure that if they went to the trouble to mix it in the oil then why should I spend the $$ on the modifier. Some claim that you should still add modifier to these oils but I ask why. If I gave you 3 pints of oil that I had added modifier to and you believed me than why would you go and add some more?


.

 

should i use the gasket they recommend? When i pulled it off factory had rtv on it...Should i use a gasket or rtv?

 

I have a Gasket on the front diff and I will never go back. The next oil change on the rear diff and I will be getting a gasket. RTV is just as good and in the long run cheaper unless you screw up its application. I just don't like the mess and the fuss of RTV.

Regards

Jean Marc Chartier