I have used the search function, and did not get the answers to my questions. Why 75W-140? Yes, I know that Ford specifies this viscosity, but it seems to me that using 75W140 in the rear is a lot like using 20W-50 in the engine. 90-weight lubes were perfectly fine for many decades, so why the change? I find it a little odd that Ford would go the 5W-20 for engine oil for fuel savings, but go to a thicker oil in the axle, which would negate the purpose of going to the 5W-20 in the first place. In replies, please do not make comments like "that's what Ford says to use," or anything like that. Ford also does not suggest modifying vehicles from stock configuration, but we swap gears, install larger tires, install performance tunes on the ECM, etc.. Almighty Ford also cursed my truck with 3.08 gears, hence my 3.73 swap.

Does anybody use a 90-weight lube in their rear axle that specifies the 75W-140? If so, which axle do you have? How long have you been using the 90-weight lube? What is your truck used for, i.e., commuting, towing, hauling?

Is anybody using 75W-110? I only know of AmsOil's offering, does anyone else offer this viscosity?

 

"Thats what Ford says to use" !

 

BlueOvalFitter, you sound like my brother!

 

Next time you tow a trailer go and touch the pumpkin. They get hot when they are worked. The higher viscosity is to keep the oil from breaking down under load.

Regards

Jean Marc Chartier

 

JMC, I understand what you are saying, but trucks survived fine with 90 weight gear lubes for decades with rear ends at the same temps.d As far as resisting breakdown, did the 90 weight lubes become a problem when Ford started going to a lifetime fill? I can see that scenario. On the other hand, a 5W-20 oil does not necessarily resist breakdown better than a 20W-50 as much as overall oil quality does. So did Ford create a safety margin for the masses in case someone fills with a sub-standard gear lube? If it is a matter of changing the 90 weight lube every 50k, I'm fine with that. I'm not a believer in lifetime fills anyway.

Please, JMC, as well as everyone else, I do not intend to disrespect. If I come across that way, I apologize. I am just trying to find the answers rather than blindly following Ford's specifications, which, while they may be great for the masses, may not be in my best interest.

 

I don't know much about anything but I'll just throw this out there, the payloads these days are much higher than even 10 years ago so the different oil might be for the extra stress on the gears from more weight. just a thought

 

BSL, that is a good point. My '02 F150 weighs as much as my old '68 F250 did. As far as tow/payload capacity, I'm not sure. We were never concerned about overloading those old Fords.

I talked with a friend that also pointed out that power levels are also up compared to years past.

Which leads me back to my theory that a synthetic 75W-90 would be just fine so long as I change the lube every 50-100k, especially since I do not tow a trailer with any kind of frequency.

 

If you want to use 75W90 go right ahead. The only time the diff gets any mention here is when it leaks, some one wants to change gears or know what gears they have or some one blows one up. It is a largely forgotten part of the truck.

Regards

Jean Marc Chartier

 

I use an 80-90 SWEPCO gear lube. Best stuff out there. Doesn't break down, period.

You'll be fine.

 

slightly off topic but mt did you switch your gears out yet?

 

I haven't. They are sitting hear. Although I had someone make an offer for them the other day. I would make $300... and then could turn around and buy a bunch more Samurai stuff. Not sure yet, still thinking about it.

 

Oddball, I have some 3.73's if you still need gearsets

 

tnsilverfx4, I did the gear swap this past October, but thank you. I would have taken you up on that deal otherwise. I will say this -- I have the standard 255/70R16 tires on it, and I still wish I had gone one step further and went with 4.10 gears. That being said, I'm not going to go back in and swap things out again. For those that are curious, my fuel economy in the city has increased from 11 mpg to 14+ mpg, and I have corrected my speedometer with a Superchips programmer and a handheld GPS. I am not sure if my highway fuel economy has increased, but I know it hasn't hurt it.