75w-90 Synthetic Gear Oil - Best Gas Mileage?

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I drained out the differential when I first bought my truck and put in Super Tech 75w-90 Synthetic Blend to run it for about 10,000 miles to clean it out before I upgrade to the big boy stuff. The question has just hit me, what 75W-90 synthetic gear oil delivers the absolute best gas mileage on the highway? What are your thoughts? I'm thinking I can't go wrong with Amsoil Severe Gear 75W-90 (SVG). Any feedback is good. Thanks!

FYI - The vehicle is a 1990 F150 2WD 5.0L V8, 3.55 gears (non limited slip) and does just fine with 75w-90, so I won't be using 75w-140.
 
I am also thinking you can't go wrong with Amsoil.
I have 75w90 in my manual transmission, and the Amsoil feels a heck of a lot better than the Redline MT-90 that it replaced.

Maybe that will help you decide.
 
I think it makes little difference, I used Mobil 1, Royal purple and Amsoil, all 75w90. I noticed no improvement in gas milelage or anything for that matter between the three.
 
I doubt you'll see any change in gas mileage. The specs on just about all store-bought synthetic 75w90s (Mobil, Castrol, Quaker State, Pennzoil, etc.) are all about the same.
 
I bet your seals start to weap shortly switching a 20 year old truck that has probably been on dino its whole life over to synthetic. I would use what you are using or a similiar synthetic blend myself.
 
Just because you have good luck with one brand in one part of your drivetrain means nothing about which one is a better product. Just happens that the amsoil product worked better in your specific manual trans. Amsoil makes a great synthetic gear oil as do Mobil 1,Valvoline,Motul,Redline,NEO..... Consider the fact that most people dont even ever drain their gear oil, so changing to most any synthetic of proper weight is very proactive on your part.
 
Originally Posted By: SLCraig
I have 75w90 in my manual transmission, and the Amsoil feels a heck of a lot better than the Redline MT-90 that it replaced.


AMSoil MTF is a clone of MT-90
 
Hi,
qdeezie - The specification is important. I ran two similar 75W-90 synthetic lubricants in the Eaton diffs in my Class 8 trucks in Interstate work

The OCIs were 1mkms (620kmls) and UOAs were taken off all trucks at regular intervals

The Castrol and Mobil products performed almost identically. I had always thought that the Castrol product - which I already knew very well - was the better - they performed the same!

The move from a 85W-140 mineral to the 75W-90 synthetic reduced operating temps by between 15 and 20C and virtually eliminated oil seal replacement

The correct spec is always important - the advertising less so!
 
EVERY vehicle I have converted to syn in the diffs has seen a very certain mpg improvement, hands down, bar none, period.

Subaru 5spd AWD. did diffs and MT. ~.5 mpg consistent.

97 Pathfinder AT. rear diff = .5mpg. AT/T-case change to syn = .5 mpg. manual hubs, 1 mpg.

2008 Jeep GC. AT change to castrol ATF+4, rear to M1 syn 75-140, somewhere between 1-1.5 mpg.

Conversely, I have never seen a mpg improvement with syn oil in the engine.

M
 
I would also vote to stay away from a full syn due to age and seals. Stick with a dino or syn blend and call it good.

My Nissan has syn from the factory, and when I replace it, I'll probably go with Mobil 1 since it's available everywhere around here and priced right!

If I were to go syn blend, the Valvoline syn blend is available everywhere here and the price is good!
 
gdeezie -
If you note a change with your new oil of the same viscosity, please let us know. I am thinking it could not be measured.
 
I changed my 4x4 from 80w-90 to 75w-90 and noticed no difference in mileage. I also tried to fill the differential to the lowest level so there would be less fluid to throw around. It made no difference. I think there might be a minute difference in mileage if you ran your vehicle on a dyno for several thousand miles, but in real life I doubt it.
 
Originally Posted By: bubbatime
I bet your seals start to weap shortly switching a 20 year old truck that has probably been on dino its whole life over to synthetic. I would use what you are using or a similiar synthetic blend myself.


Never heard of documented problems with differentials. Have you experienced this firsthand?
 
Hi,
chase - This (weepage-leaking) can happen. It is due to the cleansing action of the synthetic lubricant and the residue left at the seal face by the mineral lubricant. Some of these deposits are carbon based, some from the EP components used and sulphur etc
 
Originally Posted By: Doug Hillary
Hi,
chase - This (weepage-leaking) can happen. It is due to the cleansing action of the synthetic lubricant and the residue left at the seal face by the mineral lubricant. Some of these deposits are carbon based, some from the EP components used and sulphur etc





Is this a common occurrence or sporadic? Is it indicative of seals that are on their way out already or can functional seals begin leaking?
 
Originally Posted By: cchase
Is this a common occurrence or sporadic? Is it indicative of seals that are on their way out already or can functional seals begin leaking?


I had the opposite experience. I had a 15-year-old differential that was seeping from its seals and I drained the conventional lube and refilled it with Redline. Seeping stopped completely.
 
Last edited:
Originally Posted By: jaj
Originally Posted By: cchase
Is this a common occurrence or sporadic? Is it indicative of seals that are on their way out already or can functional seals begin leaking?


I had the opposite experience. I had a 15-year-old differential that was seeping from its seals and I drained the conventional lube and refilled it with Redline. Seeping stopped completely.


Group 5 POEs will tend to do just that.
wink.gif
 
Hi,
cchase - It can happen. The older the vehicle the more likely it will. I've had a number of occurrences in cars

In my Class 8 trucks (Eaton & Rockwell diffs) the highest distance at changeover was 750kkms (470kmls). A couple of wheel seals leaked in this case, one causing the usual costs

Synthetic lubricants were not common in diffs and manual gearboxes 20-25 years ago and some seal materials had compatibilty problems. Vehicles built after the early to mid 1990s should be OK as long as good maintenance practices were at play

Eaton for instance started promoting synthetic lubricants in the early 1990s after some years of trials. It would have been a little later in the cost sensitive cars field except for some Euro Brands
 
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