Differential fluid change - do I need friction modifier?

[155mm]

Not the FM. I was quoting the post of the owner's manual that called for a different weight diff fluid (75W-85 instead of 75W-140).

After looking around at a few forums and also the Red Line tech information summary (http://www.redlineoil.com/content/files/tech/Gear%20Oil%20Tech%20Info.pdf) it seems that the 75w140 will give you less wear at higher temperatures, while the 75w85 would give you more power due to higher friction. Since Ford says not to use any FM in the rear diff, I'm guessing they are prioritizing power over wear. The 75w-85 is also the spec'd fluid for the GT500.

The Ford product is listed as https://www.fcsdchemicalsandlubricants.com/main/product.asp?product=SAE%2075W-85%20Premium%20Synthetic%20Hypoid%20Gear%20Lubricant&category=Transaxle%20Fluid but I think you'd have to go through a dealer...I haven't found an "over the counter" 75w85 without the friction modifier, but there may be several 75w90s without the friction modifiers available. So far I've seen Red Line and Advance Auto Parts brand (but this store brand is probably a synthetic blend, not full synthetic). Lucas oil doesn't really say on their website and you can't see from the bottle picture whether their 75w90 has FM or not. Royal Purple and Amsoil both look to have FM already included.

 

[Ruger]

Since availability of gear oil with out friction modifier is limited, what drawbacks are there if you use gear oil with friction modifier in the differential? That's question number one.

Question number two is this: If you use 75w140 with friction modifier, do you get the power associated with a lighter gear oil AND the wear protection of the heaver oil?

 

[NickPic83]

Thats what I'm wondering. What could be hurt from using something with the FM already on it. Can it harm anything or is it just not needed?

Sent from my XT1080 using Tapatalk

 

[Wilson]

This kinda sounds like ethanol no ethanol! If you want heavy gear oil go to a farm store.

 

[Ruger]

Noooooooooooo, not ethanol!

 

[Ruger]

Okay, I'm back to the weird discrepancies between the various manuals for the differential gear oil. 155 posted the page from the 2013 Raptor Supplement and it specifies 75W-85. Both the 2011 F150 manual and the 2011 Raptor Supplement specify 75W-140 + 4 ounces of friction modifier. Other manuals apparently vary. I am really confused.

I don't think I can go wrong with 75W-140, but the friction modifier remains a question in my mind. Is there any drawback at all in using the friction modifier???

[As an unrelated aside, the recent bottom-of-the-screen pop-up commercial for candy when you first log on to FRF is annoying as hell.]

 

[Captain Awesome]

Okay, I'm back to the weird discrepancies between the various manuals for the differential gear oil. 155 posted the page from the 2013 Raptor Supplement and it specifies 75W-85. Both the 2011 F150 manual and the 2011 Raptor Supplement specify 75W-140 + 4 ounces of friction modifier. Other manuals apparently vary. I am really confused.

I don't think I can go wrong with 75W-140, but the friction modifier remains a question in my mind. Is there any drawback at all in using the friction modifier???

[As an unrelated aside, the recent bottom-of-the-screen pop-up commercial for candy when you first log on to FRF is annoying as hell.]

Then become a supporting member no commercials here.

 

[NickPic83]

yup supporting members don't see no stinking ads

 

[Vash]

If the manual that came with YOUR truck doesn't call for friction modifier than save your money. Same with specified oil weights. Vehicles can have even mid-year changes so I wouldn't necessarily expect different year manuals to agree. If it was a big enough change that was retroactive then I'd expect Ford to revise older manuals. You can see online that there's been manual revisions. Also, I thought that various Ford techs here already confirmed that friction modifier was for LSD diffs and not the e-locker like what's used in the Raptor.

When I get back home, I'll check my Ford Service Manual to see if it has any special guidance for the 12-9.75" rear end with e-locker.

 

[Ruger]

The 2014 Raptor Supplement (available here: 2014 Ford F-150 Owner) specifies 75W-85 premium synthetic gear oil and does not mention a friction modifier.

In complete contrast, the F150 Owner's Manual and the Raptor Supplement for my 2011 both specify 75W-140 synthetic. The F150 Manual has a footnote that says to use 4 oz of friction modifier for 8.8 inch and 9.75 inch Traction-Lok axles. The Raptor Supplement also specifies 4 oz of friction modifier with no mention of the type of axle.

So how the hell would anyone know whether the manuals that came with their vehicle are still accurate, or whether their manual has been superseded by more recent guidance???