Ok I know Ford specs out 75/140 synthetic rear axle fluid but a lot of gear companies recommend 80/90w as does a couple of local mechanics in fact one of them says he will never put in the 75/140 and only uses 80/90. What is everyones thought on what fluid to use in a rear axle.

 

Lighter oil shouldnt be used on our trucks. Big payload, high towing capacity builds huge heat and the lighter oil wont hold up to it

 

The shop who did my gear swap and then later on my Detroit locker tried to tAlk me into the ligher oil on install, told him hes out of his mind and put 75w140 back in her

 

I did 75/140 with modifier from Royal Purple because it was the cheapest I could find. On the front Dana it will be 80/90 as recommended by OE.

I don't see a need to switch weights when something has been recommended by the Manufacturer and never resulted in failure. With that said do not use MerconV in the transfer case.

 

I use mobil1 full synthetic in my tcase for six or seven years now and it hasnt exploded

 

Mobil1 is rated for several fluids. Just to be safe I recently swapped it out for the Motorcraft XL-12, which is supposed to be Mercon.

 

The covers I have taken off of diffs that specd 75-140 all looked good to me, from gears to bearings. No failures of any kind either. If it is somehow a bad weight to use I am not seeing it.

 

75-140 can see higher temps...always use synthetic. I use 80-90 synthetic in the front.

 

its the age old arguement. higher weight provides better shear loads, lower weight provides better cooling. can an axle fail if it has high shear protection and overheats....yes. can an axle fail if it has poor shear protection and runs cooler....yes. do axles fail in general...yes. pick your poision.

 

Well ford has been using and recommending 75w140 in the rear sterling axle ever since it was introduced and how many do u hear of fail? Maybe a hand few, ive had mud trucks with this axle and ive welded the spider gears together dor a backyard spool for mudding. People swap them into custom mud buggies, rock bouncers etc with nothing more than chaging the out the LS and adding at bigger yoke and they hold up. I'll continue to use 75w140

 

I think I got my answer right there..

 

wont let me post thumbs up by itself....

 

There's a case on youtube about a guy who delivers cars on a flatbed...about 4 cars at a time, total GVW of 28,000 using an f-450. in 2 years ford has replaced 4 axles under warranty. even though he is below max GVW, long term these axles cant operate at these high weights. So back to Joe blow the go to home depot and pickup some sheet rock on his f-250/f-350 on the week ends.....is he going to have axle problems....not likely.

 

What year 450 does he have? Remember Ford neutered the f450 with a lighter duty rear axle and springs not long back. Unless he has a medium duty f450, but that will have the dana110 or 135? I forget but either one is up to heavy duty work ever day. And hiw many miles is he driving? Does he change the gear oil like he should? My answer is no most likely not. And what kind of cars? He could have mini vans not a small compact car on his trailers with the heaviest vehicle right over the rear axle on the truck which is over loading it causing this so called issue. We have a 2004 F550 2wd dump at work that im in two days week and is used by the bosses son the other days of the week hauling dirt, rock, rubble etc plus towing a backhoe or tracked skid steer with ZERO issues and probably the worst maintence ive ever seen. Oil chages in the nine months ive been here are "check the dipstick and add oil if needed"

 

Hes probably over weight on the rear axle on a regular basis. A lot of these hot shot car haulers don't monitor their weights, they throw the cars on and go. Frankly if I was hauling cars on a regular basis like that I'd have an F-650 or similar sized truck.