75W-140 in a Mustang.....Why?

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Unleashedbeast

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This is a debate topic why Ford felt the need to require a 75W-140 synthetic gear oil in the (2008 Mustang) 8.8" rear diff. The car will never tow anything, obviously, and it's only 300 RWHP. Previous cars I've owned with more power only required 75W-90, so why the increase? Here are my thoughts...

1. Ford says in the owners manual that a rear diff fluid change isn't required unless the axle is submerged under water. Do they expect someone to not change the fluid despite the mileage on the fluid? Is the heavy fluid required so that it can hold up over the many miles because most people never service their rear diff?

2. Is it there to better support the rear diff in case of heavy shock loading due to hard launching at the drag strip? I'm sure Ford knows what some do to this car.

This is why I am concerned about it...

I never tow (obviously), never drag race, and rarely hard launch the car. Why do I need a heavier diff. oil that could possibly be taking away fuel economy? Since I am an Amsoil dealer now I want to change all fluids to Amsoil. What would I gain or lose from using their 75W-90 instead of the Ford recommended 75W-140?

I have my choice of either Amsoil Severe Gear oil in 75W-90, 75W-110, or 75W-140

Let the debate begin.
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The extra viscosity is supposedly extra "cushion" during launch and hold up to the high temps when open tracking as well. The 8.8 has a relatively small fluid capacity. Basically 2 Qt + the XL3 Friction Modifier, Its a very good synthetic (I believe GIV PAO Based) lube. You actually have to be pretty picky when choosing an aftermarket fluid to match the quality of the OEM Fill.

I believe under normal use its good for about 150,000 miles. If you are on the severe duty schedule I would suggest 60,000 as a reasonable change interval.
 
This is a debate thread....as to WHY Ford felt the need for such a heavy diff lubricant for a lighter car.

Let me clear up some questions out there....

1. Do I baby this car, not always, but I also don't beat it up every weekend at the drag strip.

2. I'm perfectly satisfied with my current MPG. I avg. 20 MPG city and 25 MPG hwy. If I wanted better MPG I would have bought a VW TDI Jetta.

3. My diff. is perfectly silent, no problems there.

4. This thread was nothing more than a debate as to why Ford felt the Mustang needed such a heavy diff. fluid.
 
If you want protection against shock loading but with 75W-90 levels of drag, Red Line Heavy Shockproof was made for you.

Why does Ford want such a heavy diff fluid? Probably for reduced warranty claims when they know the Mustang is going to get abused.

My Mustang spec'd 80W-90, they changed the recommendation to 75W-140 sometime after '99, '01 IIRC. I've always run 75W-90 in the rear and have never had a rear end problem other than a pinion seal leak. I'm about to rebuild it, add some 4.56s and fill it up with Heavy Shockproof.
 
Try the lighter weight oil and see what happens. It will probably be fine. They most likely specd it because they never know how hard the rear end will be abused in one of those cars. Probably they were just being cautious.
 
I see I put the word "Probably" in my answer above many times! Try looking on a Mustang forum, others must have the same thoughts as you.
 
Originally Posted By: Oldwolf
Try the lighter weight oil and see what happens. It will probably be fine. They most likely specd it because they never know how hard the rear end will be abused in one of those cars. Probably they were just being cautious.


Oldwolf is correct, they are being cautious as they don't know the conditions the car will be used in. They need to use a "shotgun" for fluid recommendations often as the car can be in Texas or the Yukon. That's why this forum exists, if you want to take your fluid use to the next level beyond the owners manual.
 
Originally Posted By: Oldwolf
Try the lighter weight oil and see what happens. It will probably be fine. They most likely specd it because they never know how hard the rear end will be abused in one of those cars. Probably they were just being cautious.


Exactly what I was thinking, they had to put the super fluid in there to insure it would survive. I still think it's overkill for my use though.
 
If it were me, and I were using Amsoil, the SG 75W-140 would be my choice. Regardless of application, I'd stick with the spec. I plan to keep HP car I buy in the future for life, so the slight lowering of MPG, if any, wouldn't bother me at all.

Off topic: It's really annoying when members chime in flaming some one for driving an his/her car the way he/she wants. It's a machine and whether the OP wants a dedicated drag car, a weekend fun car or a lively DD is irrelevant, except to determine application and give appropriate suggestions. I'm sure the OP can relate when I say I bought my WRX STI because, when I want, I can have a lot of fun on backroads, blast up to the speed limit on deserted on ramps, all the while getting up to 27 mpg doing the speed limit on long trips in a safe, AWD vehicle. I might eventually rally my car or take a HPDE course, just as the OP might, but that's not why I bought the car. In the end, these things are just machines built to get us from point A to point B just how we like it
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Originally Posted By: gathermewool

Off topic: It's really annoying when members chime in flaming some one for driving an his/her car the way he/she wants. It's a machine and whether the OP wants a dedicated drag car, a weekend fun car or a lively DD is irrelevant, except to determine application and give appropriate suggestions. I'm sure the OP can relate when I say I bought my WRX STI because, when I want, I can have a lot of fun on backroads, blast up to the speed limit on deserted on ramps, all the while getting up to 27 mpg doing the speed limit on long trips in a safe, AWD vehicle. I might eventually rally my car or take a HPDE course, just as the OP might, but that's not why I bought the car. In the end, these things are just machines built to get us from point A to point B just how we like it
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Thanks Gather, I couldn't have said it better.
 
I've never changed fluid on the mustangs I owned or the ford trucks except on a gearing change. Guess I wasn't hard enough on them. I understand breaking things makes you more manly but I stayed on street tires.
 
Hmmm, I'm considering the 75W-110 as Amsoil said it gives you an "in between" performance of 75W-90 and 75W-140

Quote:
AMSOIL SEVERE GEAR® (75W-110) provides superior performance and replaces competitors’ 75W-110, 75W- 90 and 80W-90 gear oils. It delivers better viscosity protection than SEVERE GEAR 75W-90 and better fuel efficiency than SEVERE GEAR 75W-140.


Quote:
AMSOIL INC. of Superior, WI has announced the addition of 75W-110 Severe Gear Synthetic Gear Lube to its strong line of AMSOIL Severe Gear Extreme Pressure Synthetic Gear Lubes. AMSOIL Severe Gear Synthetic Gear Lubes feature an exclusive blend of high viscosity, shear stable synthetic base oils and an extra treatment of high-performance additives. They are engineered to protect gears better than conventional gear oils, resisting acid, carbon and varnish formation and oxidative breakdown.

The new viscosity gear lube grade that fills the gap between the 75W-90 and 75W-140 viscosity gear lubes that most people are accustomed to using. Many used the 75W-140 to obtain extra film protection beyond what the 75W-90 viscosity grade offered for their differentials, but found that they sacrificed fuel economy by doing so. This new viscosity grade, 75W-110, offers a high temperature film protection beyond what a 75W-90 offers, without compromising fuel economy. This new mid-range viscosity grade for gear lubes addresses performance, protection and fuel economy.

Because today’s vehicles have seen tremendous gains in both horsepower and towing limits, greater stress is placed on differentials and drivetrain components. In fact, horsepower has increased by as much as 93 percent in turbo diesel trucks, and vehicle towing limits have increased from 7,500 to 18,000 lbs in recent years.

For this reason, AMSOIL 75W-110 Severe Gear Synthetic Gear Lube is designed to provide superior protection for elevated operating temperatures without sacrificing fuel efficiency. It is formulated for use with limited slip clutches and is ideal for turbo diesel pickup trucks, vehicles used for towing and hauling and other vehicles subject to severe service operating conditions, including heavy equipment, construction vehicles, emergency vehicles, street rods and 4x4 vehicles.

AMSOIL 75W-110 Severe Gear Synthetic Gear Lube is a premium grade lubricant specifically engineered for maximum performance in severe duty applications. AMSOIL 75W-110 Severe Gear delivers unsurpassed fuel efficiency and cold temperature performance.
 
Originally Posted By: Unleashedbeast
Hmmm, I'm considering the 75W-110 as Amsoil said it gives you an "in between" performance of 75W-90 and 75W-140


That's the one I was going to suggest you try, for the exact reasons you/Amsoil stated.
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Yes. 75W110 is what I would use. Its what I put in my 8.8.
What happened is SAE split the 90 and 140 grades in half.
The bottom half of each range retained the original number, 90 or 140. The top half of 90 became 110 and the top half of 140 became 190. This change happened around '01, IIRC. I think Ford thought the 'new' 90 range was too light. Unfortunately, 110 grades are exceedingly rare; in fact, the only one Ive found is Amsoil. So, I believe they specified 140 to ensure the weight was correct.
 
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