How can a gear oil be compatible with both GL-4 and GL-5?

[Lumberg]

Based on my limited research, the difference between GL-4 and GL-5 is the amount of extreme pressure additives. A lot of people say don't use GL-5 spec fluid in a transmission that calls for GL-4. One of the reasons given is that increased additives can cause corrosion to the brass synchronizers, but I have also since read feedback that the new additives don't have that effect anymore.

Some fluids specifically State GL-4, others specifically state GL-5, and based on my understanding, they may be mutually exclusive based on the amount of additives. However, both Notul and Royal Purple make a gear oil which is both GL-4 and GL-5. So what's the underlying truth?

 

[The Lube Guy]

HPL automotive gear oil is compatible with both.

The secret lies in the formulation.

Red Line Oil also has some gear oils that are both GL4 and GL5 compatible.

 

[Rand]

I'd prefer a single spec'ed formula.
Not a believer in CVT fluid that works in everything either.. when a manufacturer has has 3+ CVT fluids.
If they could do it all with one.. I dont think they would stock 3+

 

[The Lube Guy]

I'd prefer a single spec'ed formula.

GL4 & GL5 additive packages are more advanced (as in newer technology) than GL4 only additive packages, while GL5 only additive packages lack the synchronizer protection needed in GL4 applications. In other words, traditional GL5 formulations will damage yellow metals (e.g. erode synchronizers). Think about it this way: a manual transaxle that has a built-in differential using hypoid gears will most likely need a gear oil that is both GL4 & GL5 compatible. In fact, this isn't anything new.

 

[Rand]

GL4 & GL5 additive packages are more advanced (as in newer technology) than GL4 only additive packages, while GL5 only additive packages lack the synchronizer protection needed in GL4 applications. In other words, traditional GL5 formulations will damage yellow metals (e.g. erode synchronizers). Think about it this way: a manual transaxle that has a built-in differential using hypoid gears will most likely need a gear oil that is both GL4 & GL5 compatible. In fact, this isn't anything new.

I think you dumbed that down a little too much.

Has there been a "traditional gl-5" gear oil in years?.. . well ok lucas it turned yellow metals black... but I wouldn't use that swill.

Redline 75w90 NS for example is a GL-5 gear oil and appropriate for synchros and hypoid gears.
but if My application didnt have hypoid gear in a shared sump I wouldnt use it.. over a GL-4 MTL for example.

Let me continue .. I liked motul gear 300 which is dual rated even better in the same application. (subaru MT x4 cars)
but why should I use a dual rated gear oil in an application only requiring GL-4? In my experience it shifts worse.

Edit: for clarity

 

[bullwinkle]

My (admittedly low) experience with GL-4/-5 gear oil is RP 75W90 in the xB in my sig-it was TOO slippery & inhibited correct synchronizer operation-to the point of needing double clutching & making me think it was going to need rebuilt. Drained out the perfect looking purple gear oil, put in Pennzoil Syncromesh-it shifted like it was new!

 

[The Lube Guy]

Let me continue .. I liked motul gear 300 which is dual rated even better in the same application.

I use Motul 300 in a couple of open diffs. I will switch them to HPL gear oil very soon. Motul makes very good gear oil though.

 

[MolaKule]

Based on my limited research, the difference between GL-4 and GL-5 is the amount of extreme pressure additives.

Well, your research is not quite accurate.

GL-4 MTF's use mainly ZDDP or a ZDDP replacement since the loads within the manual transmission are less than in a hypoid differential. A hypoid differential has to use Extreme Pressure additives because of the extreme loads within the differential.

Special additive packages with low sulfur content are used in GL-4/GL-5 crossovers as well as Potassium Borates as the EP compound(s). Metal inhibitors and rust inhibitors are used to 'buffer' the additive package so as not to attack any copper-alloy or steel components.

I use the newer multi-function Phos based esters for GL-4/GL-5 gear lubes.

Synchromesh Manual Transmission Lubricants

Me too, I prefer the older chevy Burbs with the gas piston in the front diffy and the gearshifting TC, but these AUTO 4 Wheel drive Transfer cases (as in my '08 TrailBlazer) are really nice on icy and snowy roads. I recommend changing the TC fluid after a tough winter.

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