GL4 and GL5 Differences?

[jason]

What is the difference between GL4 and GL5 gear oil, in particular the Synthetic stuff? Am I right in thinking that the GL4 is meant for tranny's and GL5 is meant for rear ends?

[James]

There was a thread in here recently that talked about it - the gl-4 needs to be yellow metal safe - and what is needed for our trannies. As far as I know the gl-5 will work fine in the rear ends. Datzen Mike will chime in with an answer for sure....

[vgwagon]

What is the difference between GL4 and GL5 gear oil, in particular the Synthetic stuff? Am I right in thinking that the GL4 is meant for tranny's and GL5 is meant for rear ends?

Yes!

[datzenmike]

Many transmissions including the newer ones can use GL-5 without ill effects and so many mechanics mistakenly think that GL-5 offers more protection than GL-4 and will say so. It actually does, but it isn't needed and Japanese transmissions, at least the old ones, cannot tolerate GL-5.


Gear oils all have that sulphur smell and GL-5 oils contain a much higher level of anti-scuffing sulphur compounds needed to protect the ear surfaces. This is specially true in gear surfaces that transmit power at a sharp angle like a differential. This is not as necessary in a transmission where power is transmitted along the same axis and a lower sulphur content GL-4 oil can be used which also is less corrosive to the phosphor/bronze synchronizer rings. Typically GL-4 contains about 4% sulphur content whereas GL-5 is 6%. Doesn't look like much but this represents a 50% increase! Sulphur breaks down over time and besides providing less protection it becomes acidic and attacks copper and copper alloys also known as yellow metal. If you replace your transmission oil get GL-4 and look for copper or yellow metal friendly on the bottle. If switching to synthetic be sure it says "yellow metal safe".

[benz-tech]

Years ago I had a 280zx gearbox that I rebuilt, but in my youthful ignorance I ran GL5 in it. Even tried synthetic GL5. It shifted Ok, but I could always just barely feel the 2nd gear syncro engage while shifting. I wish i knew better then.

In my current used, albeit inspected, 280zx trans, I am running MTL-90 synthetic by, brainfart, Redline, I think?
Anyway, I have a huge need to shift very quickly all the time due to high compression, low rotating inertia and close gearing. I NEVER feel that second bump of the gear or syncro when shifing, warm or cold. I am a believer in using GL4 in the gearbox and more specifically MTL-90.

I think of all the Alfa Romeos that could have benefited from this... They should have come off the showroom floor with an extra 2nd gear syncro slid down on the shift lever before putting the knob on. That way you always would have had one handy. Maybe a 10-pack in the tool kit, could have been an option.

[jason]

I also ended up using Redline MT90 GL4 rated in the transmission. It didn't say "yellow metal safe" anywhere on the bottle, but, it was one of only 2 gear oils in GL4 at Lordco ! The other one was "Royal Purple" 75/90. I also ended up using the canadian tire synthetic GL5 in the diff. It also didn't state it was yellow metal safe, but, the idiot at the store insisted that it was fine for tranny's too ? Canadian Tire does have a blended gear oil, semi synthetic that is rated GL4 and 5 though ....

[datzenmike]

Jason I'm using the Crap Tire synthetic 'blend' in tranny and diff. Well diff now, but it is in the 620 tranny. Shifts about the same, tranny was always quiet so also no change, but, shifts better when cold or the first few shifts on a cold tranny. Also the shifter seems to make a louder 'CLICK' sound when up and down shifting which is very satisfying.

If you aren't sure about the yellow metal safe properties, keep looking and change later. Won't hurt for a short time.

[jason]

Yah, I figured as much Mike, thanks! It's gotta be better than the grey smelly stuff that came out of the diff and tranny though!

[benz-tech]

Yeah, the Redline bottle does not say yellow metal safe. But I can't imagine the engineering guys at Redline saying 'well, our MTL90 is hell on brass, but no transmissions out there use brass syncros anyway'. I think its more along the lines of 'no-one out there even knows what the hell yellow-metal is anyway, so lets not confuse them by stating its safe for it on the bottle.' It's just a hunch, but since it is Manual Transmission Lube, that it'd be OK for yellow metals. "Yellow metal friendly" seems to me like saying gasoline is "safe for combustion engines"
But what do I know anyway

[hang_510]

The other one was "Royal Purple" 75/90.

i just changed this out of the diff to valvoline (both for LSD)

the RP seemed to seep out the vent after any cyn driving.
after the first trip since the change, no more mess.