('03-'05) Rear Diff Fluid...GL4....why not? - Subaru Forester Owners Forum
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post #1 of 15 (permalink) Old 05-17-2017, 11:23 AM Thread Starter
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Rear Diff Fluid...GL4....why not?

I like to play games and mess with certain fluids even at the expense of reliability where it's possible to reduce driveline losses. Not many in the Subaru community seem to have any interest in doing so which makes my questions astronomically harder to answer.

I have done alot of searching here and it seems that GL4 is a no-no in the autos
front diff. I accept that.

However it *seems* that the rear diff is not nearly as picky. I have some Redline GL5 75w-85 gear oil that I am probably going to chance.

However, I also have some MTL laying around and that is 75w-80....but it is GL4.

Anyone know if GL4 is ok in the rear? I personally think that MTL is a bit thin, but like I said, I like to play and Redline is great fluid.

FYI, the vehicle is fundamentally driven easily on the highway with MPG in mind....but I'd still like to reduce driveline losses.

Thanks.


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post #2 of 15 (permalink) Old 05-17-2017, 11:40 AM
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Believe GL4 is not compatible with certain metals ( can't remember which ones) that GL5 is compatible with, so there's a fair chance the formulation used in GL4 will destroy a bearing component or some other specific metal in your diff. Surely not worth the risk?

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post #3 of 15 (permalink) Old 05-17-2017, 11:52 AM Thread Starter
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I know that the inverse applies to my Mitsubishis not being able to use GL5 in the trans for instance due to brass synchros....not the same situation but just saying.

If it is immediate death to the component it is not worth the risk, no. If it is more of a subjective argument I am game.

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post #4 of 15 (permalink) Old 05-17-2017, 11:55 AM
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I don't think your diff is a viscous or clutch type (which would have exotic materials which might require a certain fluid), so you can likely get away with nearly anything, because its all steel in there. I've always considered keeping the fluid clean is more important than its exact chemistry or viscosity (within reason, of course). I have a really nasty gear rattle in my BMW transmission because I swapped the dual mass flywheel out for a lightweight unit, so I put 85-140 in to quiet it down. I put cheap oil in my suby gearcases (because I had the transmission apart every two weeks for awhile there), and likely will continue to, just changing it every year.

Go for it. Report back.
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post #5 of 15 (permalink) Old 05-17-2017, 12:01 PM Thread Starter
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The rear diff is a sealed viscous unit based upon my understanding.

Yep, within reason. I am familiar with going heavier too under the right circumstances. My 3 bolt rear in my Mitsubishi is much weaker than but much more efficient than the 4 bolt rear. In that instance, I went with Redline Shockproof heavyweight and still came out ahead.

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post #6 of 15 (permalink) Old 05-17-2017, 03:27 PM Thread Starter
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I did the Redline 75w-85.

https://www.redlineoil.com/product.aspx?pid=40&pcid=4

It has actually quieted my rear diff. Too soon to tell what things will be like long term. Who knows what was in there before. Sounded like a wheel bearing was going bad at speed. I guess it wasn't. The sky hasn't fallen so far.

Thanks for the thoughts.

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post #7 of 15 (permalink) Old 05-17-2017, 05:42 PM
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Quote:
Originally Posted by 2004XT_Auto View Post
I know that the inverse applies to my Mitsubishis not being able to use GL5 in the trans for instance due to brass synchros...

.
NIssan MT's , too, need GL4, as GL5 extreme pressure additives are corrosive to the synchros .

GL5 and GL4 aren't interchangeable.
I would follow the spec only.
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post #8 of 15 (permalink) Old 05-18-2017, 04:22 AM Thread Starter
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Originally Posted by hillbille View Post
NIssan MT's , too, need GL4, as GL5 extreme pressure additives are corrosive to the synchros .

GL5 and GL4 aren't interchangeable.
I would follow the spec only.
Yes, I went GL5 but thinner at operating temperature. I believe some of the new Toyota/BRZ use GL3, so things are all over the place.

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post #9 of 15 (permalink) Old 05-18-2017, 04:49 AM
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Quote:
Originally Posted by 2004XT_Auto View Post
I like to play games and mess with certain fluids even at the expense of reliability where it's possible to reduce driveline losses. Not many in the Subaru community seem to have any interest in doing so which makes my questions astronomically harder to answer.

I have done alot of searching here and it seems that GL4 is a no-no in the autos
front diff. I accept that.

However it *seems* that the rear diff is not nearly as picky. I have some Redline GL5 75w-85 gear oil that I am probably going to chance.

However, I also have some MTL laying around and that is 75w-80....but it is GL4.

Anyone know if GL4 is ok in the rear? I personally think that MTL is a bit thin, but like I said, I like to play and Redline is great fluid.

FYI, the vehicle is fundamentally driven easily on the highway with MPG in mind....but I'd still like to reduce driveline losses.

Thanks.
It depends. If you have an open or helical gear driven diff, stick with GL5.
If you have a viscous limited slip rear. Ignore me.

Quote:
Originally Posted by CCRIEN View Post
Believe GL4 is not compatible with certain metals ( can't remember which ones) that GL5 is compatible with, so there's a fair chance the formulation used in GL4 will destroy a bearing component or some other specific metal in your diff. Surely not worth the risk?
GL4 vs GL5. For the most part, vary only by the amount of phosporous and sulfur additives. GL5 having more. Corrosive is the wrong word to use... Yellow metals sitting in a path of GL5 will not corrode...

The Phosphorous and sulfur additives in high temp/pressure scenarios like seen in gear on gear, tooth on tooth, pressure under engine load react, and bond momentary sacrificial layers to protect the steel gears. When this reaction/ bond layer occurs on softer, yellow metals, syncrhos, and is sacrificed bits of yellow metal go with it... it eats away at it.

GL5 has the highest level of these additives, to provide the best protection. Great for systems that are all steel on steel action. Differentials.

GL4 has a more moderate level of these additives, to extend the life of synchros in a syncrhomesh transmission.

It really wigs me out that subaru combines the front diff and transmission for their manuals, rather than separate like with their CVT's. Choosing the correct fluid wierded out my OCD. This contributed to some extent of going CVT on my second Subaru, and first slushbox in many years.
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post #10 of 15 (permalink) Old 05-18-2017, 05:06 AM
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Quote:
Originally Posted by Menel View Post
. Yellow metals sitting in a path of GL5 will not corrode....., and is sacrificed bits of yellow metal go with it... it eats away at it.

.
Uh........... it seems like you are contradicting yourself.


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post #11 of 15 (permalink) Old 05-18-2017, 05:18 AM
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Quote:
Originally Posted by hillbille View Post
Uh........... it seems like you are contradicting yourself.
It's not corrosion. You can sit yellow metals in a bath of GL5, and it will be fine months, years later.

A manufacturer can market a bottle of GL4/5 gear oil and state truthfully and lawfully "will not corrode yellow metals".

The reaction of the phosphorus and sulfur in high pressure scenarios; however, will peel way at the yellow metals...

.
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post #12 of 15 (permalink) Old 05-18-2017, 06:08 AM
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Originally Posted by 2004XT_Auto View Post
I like to play games...even at the expense of reliability
Your main premise makes any discussion moot as you're not coming from a sound starting point. Pure fool's errand. There is a good reason why it's hard to find like-minded people doing the same.

You want to reduce driveline loss in such a miniscule amount as caused by fluids? You'd be better served by skipping breakfast or throwing your floor mats out.

But, hey, it's your car. You have the right to destroy it if you want. Godspeed.
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post #13 of 15 (permalink) Old 05-18-2017, 08:20 AM Thread Starter
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Originally Posted by sneefy View Post
Your main premise makes any discussion moot as you're not coming from a sound starting point. Pure fool's errand. There is a good reason why it's hard to find like-minded people doing the same.

You want to reduce driveline loss in such a miniscule amount as caused by fluids? You'd be better served by skipping breakfast or throwing your floor mats out.

But, hey, it's your car. You have the right to destroy it if you want. Godspeed.
I don't think that your comment is sound actually. As a matter of fact, I was going to share the fact that Redline themselves just responded with 75W-85 GL5 being ok.

I'd think that they'd know a thing or two?

My comment was not an insult to the entire Subaru community. There are just philosophical differences between the Mitsu and Subaru community that I am not used to. To me, the enjoyment in modding a vehicle comes down to not just individuality but the cumulative effect of what you have done to a vehicle. Skipping breakfast and floormats, hey why not go with lighter fluids as well? Before I know it, I'll have one of the quickest TD04 equipped 4EATs. This is how it's done elsewhere...obsession and focus go hand in hand with a competitive edge.

I doubt very much that I am going to destroy my car by going with slightly thinner fluid. What an arrogant, bald, and cliche statement.

If my rear diff blows up soon, I'll be back to report it. I'd think that the community would be happy to have data on what happens if you take things "too far". I know that I enjoy reading what not to do after someone has "destroyed it". Not to say that I enjoy that person blowing it up....but better them than me.

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post #14 of 15 (permalink) Old 05-18-2017, 08:48 AM
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Quote:
Originally Posted by 2004XT_Auto View Post
I don't think that your comment is sound actually. As a matter of fact, I was going to share the fact that Redline themselves just responded with 75W-85 GL5 being ok.

I'd think that they'd know a thing or two?

My comment was not an insult to the entire Subaru community. There are just philosophical differences between the Mitsu and Subaru community that I am not used to. To me, the enjoyment in modding a vehicle comes down to not just individuality but the cumulative effect of what you have done to a vehicle. Skipping breakfast and floormats, hey why not go with lighter fluids as well? Before I know it, I'll have one of the quickest TD04 equipped 4EATs. This is how it's done elsewhere...obsession and focus go hand in hand with a competitive edge.

I doubt very much that I am going to destroy my car by going with slightly thinner fluid. What an arrogant, bald, and cliche statement.

If my rear diff blows up soon, I'll be back to report it. I'd think that the community would be happy to have data on what happens if you take things "too far". I know that I enjoy reading what not to do after someone has "destroyed it". Not to say that I enjoy that person blowing it up....but better them than me.
I'm so confused... of course RedLine said GL5 is ok...

You were asking about GL4...

.
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post #15 of 15 (permalink) Old 05-18-2017, 08:53 AM Thread Starter
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Originally Posted by Menel View Post
I'm so confused... of course RedLine said GL5 is ok...

You were asking about GL4...
I mentioned that I might try 75w-85 GL-5 in my initial post even though it had a lower viscosity than recommended. I didn't go with 75w-80 GL-4 because I figured that was outside of the GL spec but also the viscosity spec and I was right. It was just too far out.

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