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75w90 vs 75w140 differential Oil

22K views 11 replies 8 participants last post by  Herb07GT 
#1 ·
I know that is a topic from the paleolithic. But I want to make some questions and reflections.

Ford recomended the 75w140 for our mustangs, but I read that in 2015 mustangs, the recomendation is 80w90 (or 75w90 not remember).

I have read several post in forums, and the pros of 140 grade vs 90 are: less noise, better protection on track and towing because the hight temperature reached

However, 90 grade pros are: better fuel economy and HP (1-2%), less friction loss and less temperature at the same velocity which is good for the bearings.

I measured the temperature of the differential at 25ºC (50-70mph) and was 150ºF (65ºC) with the original oil.

In this paper(only abstract and 5 pages availables) the autors compare Ford 75w140 oil with others 75w90 and 75w140 oils.

Fuel Economy Improvement Through Frictional Loss Reduction in Light Duty Truck Rear Axle

Some conclusions are that the composition(syntetic or not) of the oils and their additives are more or as importants as the viscosity grade when considering fuel economy, axle temperature and axle efficiency.

In advance, when a 75w140 oil with high VI (190) and low viscosity at 40ºC is tested the resuls are comparables to others 75w90 and better than the 75w140 oil of lower VI and higher viscosity at 40ºC

Do you think that is possible to use an 75w90 oil in street drive and ocasional sport drive withouth issues in a Mustang? otherwise, any 75w140 quality oil? Here in Europe Motul, Castrol or Ravenol are options that I am rewiewing. Some sugesstions?

One doubt. One oil of a famous brand that affirm that is syntetic, has a pour point of -36ºC...is this possible or can be an error in the tech doc?. Other oils that are PAO based with the same viscosity grade are -45º to -50 ºC (pour point)

Thanks
 
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#2 ·
An improvement in axle efficiency up to 4.3% was observed over current Ford factory fill SAE 75W-140 lubricant depending on speed, torque and the type of lubricant used.

Chassis roll fuel economy tests conducted using a 5.4L V-8 full size sport utility vehicle (SUV) showed up to 1.4% improvement in the Federal Test Protocol (FTP) metro/highway (M/H) cycle
The difference is too small to be of any significance, and the oil recommendation is different in 2015 Mustangs because they have IRS instead of a solid rear axle.
 
#3 ·
. . . Do you think that is possible to use an 75w90 oil in street drive and ocasional sport drive withouth issues in a Mustang? . . . .
Yes -- especially if it is one of the high end synthetics.

And thanks for posting that information; there are lots of opinions floating around about this, but this is the first post I remember that had some real data.
 
#4 ·
I went with Mobil 75W-140 with Lucas mixed in for mine since longevity and possible HDPE events are goals. It's what I did in the past on my Dodge truck and after 200k miles, the rear end was the only part of the drivetrain I hadn't had issue with. Being that this is an 8.8, I'd stick to Mobil or Lucas synthetic and won't be surprised if I see 300k+ out of the rear end.
 
#5 ·
If you have a clutch type limited slip rear end (as opposed to a Torsen or open design) you need to stick with the 75w-140 as this weight gear oil has the required friction modifiers formulated in. The lower viscosity 75w-90s don't automatically include the friction modifiers and I'm not sure if the modifiers are available as an additive or not.

I think all GTs come equipped with the limited slip rear ends with the Torsen being an optional upgrade (part of the track pack?). I think the V6 models have open rear ends as standard. No clue what the Ecoboost uses.
 
#6 ·
you can buy the friction modifier separately, it is readily available
 
#7 ·
That's what I did. Ordered a bottle of FRPP Friction Modifier even though the Mobil & Lucas both included. Rear end is perfectly smooth and quiet.
 
#8 ·
Thanks for your answers.

The cause of this topic is that in curves at low speed (parking), my rear axle shudders or chatters (clack clack) and it seems that can be the LS differential. So I've done a little research, and I was very confusing because the web overinformation

It seems that "good quality" 75w140 oils (high VI, low viscosity at 40ºC and syntetic) are similars to 75w90 oils in fuel economy, axle temperature, etc. So we should not worry about excessive penalties respect to 75w90 oils.

The question is that without information of the oils composition, additives...and laboratory test, to decide if this oil brand is better than another is difficult. Moreover in LS differentials, the friction modifier is also decisive and it seems that it degrades over time.

Brand not reveals much information about composition and test. When a brand assures that one oil increases fuel economy...perhaps is 1% or less. Not noticeable for a daily driver, although is significant if you are owner of a truck fleet.

I have search this book in google.books. Writing in google, it appers the volumens I to III
Handbook of Lubrication and Tribology: Volume I Application and ..., Volumen 1

Axle lubrication in chapter III

Thanks
 
#9 ·
I've owned older cars as a hobby, and one of them called for SAE 140 gear oil (GL4) in the rear axle and the gearbox. I found it hard to shift gears when it was cold. I ended up using 75W-140, and the car did not appear to suffer any undue wear or deterioration of the gears or bearings over a period of about 20,000 miles, at which time I sold the car.

I believe that how the car is used is important in making this decision. If it is not being tracked, or towing heavy loads, or being driven hard, the lower vis oil should be ok.
 
#10 ·
I picked up this kit when I changed the fluid in my rear differential, the difference was extremely noticeable for me. It is very smooth now when I am cruising on the freeway, feels like a whole new car. It is worth noting that I don't think the previous owner (female friend of mine), EVER had it changed out..so yeah

https://lmr.com/item/LRS-F8805K/88-Rear-Differential-Fluid-Seal-Kit-Mustang-05-14?year=2007&gclid=Cj0KEQjw_eu8BRDC-YLHusmTmMEBEiQArW6c-PjF4SsFHTud3qoGuLgWIl3XWv_Y6v8H7M5p3R8z61YaAm418P8HAQ
 
#11 ·
Hello again

I have consulted with an expert in lubrication and told me basically what we have exposed.

75w140 if hard acceleration is frequent (most probably) or towing
75w90 at low temperatures, or if hard aceleration is infrecuent (unlikely)

Keep in mind this chart of viscosity vs temp. Below 40ºC-100ºF the viscosity of a 75w140 oil is far superior than 75w90 oil althought both are 75w (it is strange for me)

Grafico viscosidades: Diferenciales

Blackout1231, is curious that Ford Racing FM is recommended when the RP85w140 cointains FM. Some people point out that excesive FM may cause excesive slip, but also others suggest to use more additive if Limited Slip noise is heard. Everyone decides.

This may be another topic: experiences about use more adittive that the recomended, worst or better.


Thanks
 
#12 ·
Great Topic - Worth the read!
 
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