Differential fluid changes, and friction modifier question

[jcm3]

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.

[work&play]

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.

[skanj0]

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).

[mazsecond]

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.

[jd4010]

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.

[jcm3]

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.

[JIM_C]

Originally Posted by mazsecond

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.

[jcm3]

I think I finally found some good information to answer my question, and maybe it will help others. I kept searching with different keywords and phrases,

Will It Hurt if You Use a Friction Modifier in a Non -Limited Slip Axle? | eHow.com

mobil 1 synthetic gear oil 75w-90 ls in open diff? - Bob Is The Oil Guy

[Boreal]

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.

[jcm3]

Originally Posted by Boreal

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.