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#2057475 - 10/25/10 06:09 PM 75w-90 Synthetic Gear Oil - Best Gas Mileage?
qdeezie Offline


Registered: 08/10/10
Posts: 1248
Loc: Charlotte, NC
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.

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#2057478 - 10/25/10 06:10 PM Re: 75w-90 Synthetic Gear Oil - Best Gas Mileage? [Re: qdeezie]
SLCraig Offline


Registered: 12/21/08
Posts: 5627
Loc: London, ON, Canada
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.
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#2057493 - 10/25/10 06:16 PM Re: 75w-90 Synthetic Gear Oil - Best Gas Mileage? [Re: qdeezie]
salesrep Offline


Registered: 08/22/03
Posts: 3391
Loc: lllinois
75w-90 Synthetic Gear Oil - Best Gas Mileage?
Ones with friction modifiers!
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#2057532 - 10/25/10 06:42 PM Re: 75w-90 Synthetic Gear Oil - Best Gas Mileage? [Re: qdeezie]
Panzerman Offline


Registered: 12/16/06
Posts: 3401
Loc: Port Orange, Florida
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.
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#2057548 - 10/25/10 06:54 PM Re: 75w-90 Synthetic Gear Oil - Best Gas Mileage? [Re: qdeezie]
dparm Offline


Registered: 04/19/10
Posts: 13040
Loc: Chicago, IL
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.
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#2057583 - 10/25/10 07:23 PM Re: 75w-90 Synthetic Gear Oil - Best Gas Mileage? [Re: qdeezie]
bubbatime Offline


Registered: 03/18/08
Posts: 4127
Loc: South Florida
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.
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#2057707 - 10/25/10 09:05 PM Re: 75w-90 Synthetic Gear Oil - Best Gas Mileage? [Re: SLCraig]
AzFireGuy79 Offline


Registered: 03/08/08
Posts: 1286
Loc: Arizona
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.
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#2057858 - 10/26/10 12:09 AM Re: 75w-90 Synthetic Gear Oil - Best Gas Mileage? [Re: SLCraig]
Audi Junkie Offline


Registered: 11/03/02
Posts: 11247
Loc: PA
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
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#2057897 - 10/26/10 01:07 AM Re: 75w-90 Synthetic Gear Oil - Best Gas Mileage? [Re: qdeezie]
Doug Hillary Offline


Registered: 05/30/03
Posts: 5102
Loc: Airlie Beach Australia
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!
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#2058020 - 10/26/10 08:02 AM Re: 75w-90 Synthetic Gear Oil - Best Gas Mileage? [Re: qdeezie]
meep Offline


Registered: 02/20/07
Posts: 3096
Loc: Southeast
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
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#2058122 - 10/26/10 09:32 AM Re: 75w-90 Synthetic Gear Oil - Best Gas Mileage? [Re: qdeezie]
K20FA5 Offline


Registered: 03/03/10
Posts: 390
Loc: Prosper, TX
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!
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#2058556 - 10/26/10 03:01 PM Re: 75w-90 Synthetic Gear Oil - Best Gas Mileage? [Re: qdeezie]
mechtech2 Offline


Registered: 09/05/06
Posts: 19479
Loc: Chicago Area
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.

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#2058841 - 10/26/10 06:06 PM Re: 75w-90 Synthetic Gear Oil - Best Gas Mileage? [Re: qdeezie]
Loobed Offline


Registered: 07/20/07
Posts: 3742
Loc: Northern California, USA


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.
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Current Fill: Valvoline Synpower 5w-30 w/Pure-One PL25288 filter

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#2058848 - 10/26/10 06:11 PM Re: 75w-90 Synthetic Gear Oil - Best Gas Mileage? [Re: bubbatime]
cchase Offline


Registered: 11/03/09
Posts: 4021
Loc: New England
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?

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#2058861 - 10/26/10 06:18 PM Re: 75w-90 Synthetic Gear Oil - Best Gas Mileage? [Re: cchase]
Doug Hillary Offline


Registered: 05/30/03
Posts: 5102
Loc: Airlie Beach Australia
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
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Doug

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