Hi. I'm about to change rear differential fluid for my father's 2002 F-350 and I'm very confused about friction modifier requirements. Google search just returns with all different answers and cannot decide what is true and what is not.

The manual specifies 75W-140 Synthetic with 4oz of friction modifier. When I looked up 75W-140, all are synthetic and says for LSD, which suggests they come with friction modifier already in them. Choices I have from Advance Auto Parts are Valvoline, Royal Purple, and Mobil1.

From what I searched on google, some says additional friction modifier is not needed, some says must use genuine Motorcraft 4oz, some says 4oz is not enough and need 2 bottle.

What's with all this confusion?

What should I get?

Below is tag on the differential if that helps.








Another question I have is, I found that there is gasket you can use for this differential. Gasket would make the job much easier and faster than RTV since I don't have to wait for it to cure and don't have to worry about oil residue on mating surface. Will there be any issue with using gasket instead of RTV?


The truck has 230k and never changed differential fluid. Hope it still works ok...

 

Hard to read the bottom tag so cant determine if LS or open diff. If open diff, no modifier is needed. Looks like the first letters are l73 which likely means 3.73 LS though.

I used royal purple on our 06 LS axle and vacillated as to whether to add modifier since the literature indicates it already has it added to the oil. I didnt add any extra and it seems fine with no chattering evident.

I also thought about using a gasket but went the RTV advesive route and let it cure overnight. Didnt want to spend $25 for a simple paper gasket.

You might also consider adding an 08 finned alum cover as they are pretty cheap and have a machined flange making oil leaks without a gasket less likely.

 

2nd line of bottom tag starts with 3L73 10 5 and from what I searched, 'L' means Limited Slip and 3 73 means 3.73 axle ratio and 10 5 means 10.5" ring gear.

What was your differential? May be I will go for Royal Purple without friction modifier as well.

For the age and condition of my father's truck, upgrading differential cover probably is a waste. Body of the truck has started to rusting hole through. Parking brake backing plate has rusted and broke mounting hole. Planning on replacing it at the same time I replace differential fluid.
Brake line has ruptured on the road from rust once, mechanic replaced only half of the line with incorrect type of coupling (used compression union). I decided to replace rest of the line, which broke during removal. (good thing I replaced)

 

+1 to royal purple working. I dumped in my 07 half ton with no modifier it has limited slip and no chattering at all.

 

+1 on the royal purple as well. Use it in all of my trucks and off-road mudding trucks. Stuff is great. Has friction modifier already added so you don't have to worry about adding it. When changing diff fluid pull off the cover clean it with scotch bright pad put a rag over the gears and clean that edge a well. No need to really wait for it to cure. Apply the rtv bolt up the cover and fill rear diff. No big deal.

 

Anyone have experience with Mobil1 or Valvoline?
Advance Auto Parts have $2 discount going on for Royal Purple, which means I cannot use one of those 20-25% off on-line coupon for additional discount. I would like to save as much as I can.

Just found out front axle seal is leaking too, but too much work to replace. Probably will just top off the fluid until it gets worse. Don't get to use 4x4 often (may be once or twice a year), so hopping it won't give much problem.

Don't I have to wait at least about an hour before pouring oil?
Gasket was only $6 at advance auto parts.

 

I am using Mobil1. I am not using any additional friction modifier. It feels very smooth and the limited slip functions fine without it.

 

I'd be interested in a good source for that cover and those bolts as well, thanks!

 

WRV- here are a couplle links for the cover, do not buy the hardware at Ford. It is way overpriced. Instead go to a hardware store and get some flange bolts.

WWW.HORIZONPARTSGUY.COM



REAR COVER | Genuine Ford | 8C3Z-4033-A

 

I went with Amsoil Severe Gear oil and initially did not add friction modifier. I got some chatter, primarily when turning hard and low speeds loaded. I added 2 oz of slip lock and still have some chatter but have been dealing with it. Too much modifier makes the LSD even more worthless than it already is.

 

If you use a synthetic fluid do not add friction modifier until you have gone out to see how the differential reacts. If you have some binding during a turn on pavement, add a small amount and keep trying until the binding is gone.

Most synthetic fluids do not require friction modifier added and if you do add it, you are wasting your time and money because you will turn your LSD into a LGD (limited grip differential).

 

Thanks for the advises. I will go for Mobil1 without friction modifier. There is $40 off $100 coupon code at advance auto.
Even after that, it is adding up to $71 for front and rear. 75W-90 for front is much cheaper at $12/qt, while rear 75W-140 is $20/qt.

It is 3.5 qt for ear and 2.5 qt for front if remember correctly. Right?

 

Thanks! Reps sent.