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Discussion Starter #1
I have been doing my research, and would like to change out the front and rear diff fluids with a good synthetic gear oil. I was thinking about going with Royal Purple, but then read about their use of Moly, and will probably avoid them for that reason.

My question has to do with the front diff fluid. 80w-90 is recommended, but friction modifier is not needed. It seems that most of the synthetic gear oils (Royal Purple, Redline, Mobil 1) all contain friction modifier, though. Is it still ok to run them in the front? I would assume since I need to lock the hubs, the friction modifier won't hurt anything?

I'll use 75w-140 synthetic in the rear, and will only add additional friction modifier, if needed. (not likely since it already has it)

Thanks for any input. :icon_ford:
 

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I put Mobil 1 in my rear diff. and seams good.
This is the 2nd time I have changed it and used the Mobil 1.
I need to do my front this winter.
 

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You can get Redline oils in both weights without the friction modifier. This is what I did. Then add friction modifier to the rear axle only if needed (assuming you have a limited slip).
 

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I would stick to what Ford recommends for the diffs. As for the front you will want to use 5.9 Pints or 2.8 Liters of Ford Motorcraft SAE 80W-90 Premium Rear Axle Lubricant with no friction modifier. For the rear you will want to use 6.9 Pints or 6.3 Liters of the Ford Motorcraft SAE 75W-140 Synthetic Rear Axle Lubricant. If you DO NOT have limited slip you DO NOT have to add any frication modifier but if you DO have limited slip you will have to add some of the Ford Motorcraft Additive Frication Modifier (XL-3). Ford recommends adding 4 ounces of it but I would start with 2 ounces and add some more if needed. Ford recommends adding more just to limit noise coming from the axle on turns.
 

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When I replaced the diff fluids in my '04 350 DRW, I replaced with AMSOIL Severe Gear 75-140 for the rear and Severe Gear 75-90 for the front. I purchased the Limited Slip modifier as well but didn't use any. I'm a believer that you should replace the oil and test drive it first to see if the modifier is needed before just dumping some in there.
 

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Discussion Starter #6
Yep, I think we're all on the same page regarding the use of friction modifier in the rear. My main question is if the synthetic stuff with it already added will harm the front diff in any way.

mazsecond, I may end up just going with Ford stuff in the front, but an curious to what others are doing. I'd prefer to go with a good synthetic oil, if possible.

skanj0, I didn't know the Redline stuff came without it, I'll have to check that out.
 

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I would stick to what Ford recommends for the diffs. As for the front you will want to use 5.9 Pints or 2.8 Liters of Ford Motorcraft SAE 80W-90 Premium Rear Axle Lubricant with no friction modifier. For the rear you will want to use 6.9 Pints or 6.3 Liters of the Ford Motorcraft SAE 75W-140 Synthetic Rear Axle Lubricant. If you DO NOT have limited slip you DO NOT have to add any frication modifier but if you DO have limited slip you will have to add some of the Ford Motorcraft Additive Frication Modifier (XL-3). Ford recommends adding 4 ounces of it but I would start with 2 ounces and add some more if needed. Ford recommends adding more just to limit noise coming from the axle on turns.
Assuming that's a typo re: liters because 6.9 pints is a bit over 3 liters. I used about 3.5 quarts in my rear diff, including 2 oz of friction modifier. It works better now than it did from the factory.
 

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Discussion Starter #8

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Those are good links and what I kind of suspected anyway. It seems like many off-the-shelf gear lubes have it, and I know they're being used in open differentials all the time. If there was indeed a problem with running the modifier in an open dif, I think you'd be hearing stories about it all day long.

I bought some Napa branded stuff yesterday for my wrangler... it has the modifier in it.
 

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Discussion Starter #10
Those are good links and what I kind of suspected anyway. It seems like many off-the-shelf gear lubes have it, and I know they're being used in open differentials all the time. If there was indeed a problem with running the modifier in an open dif, I think you'd be hearing stories about it all day long.

I bought some Napa branded stuff yesterday for my wrangler... it has the modifier in it.
Yeah, that was my thinking also. I had to check tons of links to find one where it said it was ok to use, but didn't find much at all that said that it would be, or was, a problem using it in an open diff.
 

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skanj0, I didn't know the Redline stuff came without it, I'll have to check that out.
I think I ordered mine from Summit Racing.
 

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Just read through this... I believe my rear diff says to use synthetic lube only so it's what came in it from the factory.


Sent from my Autoguide iPhone app
 

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My rear axles is "spec'd" to be synthetic fluid only also. I have the Royal Purple in mine. I have a limited slip rear, but didn't add any extra friction modifier than what is alread in the Royal Purple. Never heard of the friction modifier doing any harm to a non limited slip differential. Oh yeah....so.....what's the problem with the moly in diff fluids and why is it bad? I must've not heard about this problem.
-Aaron
 

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Discussion Starter #15
My rear axles is "spec'd" to be synthetic fluid only also. I have the Royal Purple in mine. I have a limited slip rear, but didn't add any extra friction modifier than what is alread in the Royal Purple. Never heard of the friction modifier doing any harm to a non limited slip differential. Oh yeah....so.....what's the problem with the moly in diff fluids and why is it bad? I must've not heard about this problem.
-Aaron
It could be overblown, but I came across a few links when doing some searching. Here is one link from this site: http://www.powerstroke.org/forum/general-6-0l-discussion/134147-royal-purple.html

I put Royal Purple in the diff of my rv, and know that it is good oil. But, there are so many good other brands also (Mobile 1, Redline, Amsoil, etc.) that if there is any controversy, I would just assume go with one of the others. :dunno:
 

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if u buy the modifier do not open the bottle unless your going to use it

it is the worst smelling stuff ever

and it seems to creep out of the bottle one opened ,, I can smell it even through the ziplock bag I put it in
 

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Hmm. Interesting. I was actually considering switching over to RP due to an acquaintance that had a 900 (now 1200ish) hp daily driver/track Cummins that uses RP and he just tore it down after a good amount of abuse and the inside of the motor looked perfect. Guess a little more research is in order. I'm not too worried about it though. Oil is an easily debated topic. I can find failures on dang near any oil that is sold.
-Aaron
 

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Discussion Starter #18
Hmm. Interesting. I was actually considering switching over to RP due to an acquaintance that had a 900 (now 1200ish) hp daily driver/track Cummins that uses RP and he just tore it down after a good amount of abuse and the inside of the motor looked perfect. Guess a little more research is in order. I'm not too worried about it though. Oil is an easily debated topic. I can find failures on dang near any oil that is sold.
-Aaron
Yep, I agree with you on that.
 

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I have been doing my research, and would like to change out the front and rear diff fluids with a good synthetic gear oil. I was thinking about going with Royal Purple, but then read about their use of Moly, and will probably avoid them for that reason.
RP is far from the only company to use Moly.... Valvoline, Delvac, Delo, John Deere and Schaeffer's all use it. The issue becomes which type of Moly do they use...

The moly in your motor oil and the moly is your gear oil isn't the same thing. Gear oil usually uses molybdenum disulfide, while motor oil uses molybdenum dialkyldithiocarbamate.

A lot of the worry comes from a Cummins bulletin referenced here..

Dodge Cummins Diesel Forum - View Single Post - Oil report help

But that bulletin references molybdenum dithiophosphate, not molybdenum disulfide or molybdenum dialkyldithiocarbamate.

Personally, I don't know what RP uses and it is way to expensive for me to care or find out. I have seen UOA's of RP getting sheared down from a 140 wt gear oil to a 90 wt gear oil so that is reason enough for me to avoid it, not their use of Moly.

Personally, I just stick with an name brand (Mobil, Motorcarft, Valvoline) and you'll be fine IMHO.
 

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Discussion Starter #20
Thoughts on Redline?
 
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