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.
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.
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.