GL-4 or Higher...?


Active member
Hi All,
I'm finishing up the fluid change in my 89. I have GL-5 80W-90 on hand have have read up on the differences between GL-4 and GL-5 and that GL-5 may not be the best for yellow metals within a transfer case, but I'm confused by the statement in the FSM stating "API Classification GL-4 or higher". What has me confused is the "or higher" statement. I searched within the forums and couldn't really find any recent discussions on the topic. While not wanting to start another oil war, does anyone have some input?
all true, kinda.
use caution and go with 4.
the common explaination, and I'm unsure of its accuracy is that 5 is too slippery for many synchro rings. there is a point where they fail to do their job because of this, essentially grinding themselves to death.
this has been a common problem on zf 5 speed with the original synchros, and lubes were blamed.
I have dozens of years professional association with lube providers.
the big labs I have grown to trust extensively, the boutique labs, less so.
generally because they did not deliver proportionate to their increased cost.
if u have any doubt at all, then use the exact recommended fluid. the only acceptable deviation from this would be a synthetic blend of exactly the same grade.
especially for the transfer case, a synthetic or blend will free up a lot of parasitic drag.

hope that is helpful.
furthermore, I have learned to use Japanese blended oils in my mitsubishi.
the effect is instant and profound.

for a long time I have banged the drum for Wal mart oils based on lab tests. and I still tout them as superior to many branded oils.

one oil change with eneos taught me that not all oils are the same, oh no. in the case of Japanese metals, I am firm in my convictim that Japanese metal and oil need to go together.

try to source either mitsu or eneos branded oils. there u will likely find a proper blend and an oil guaranteed to be compatible with ur machine.


Expedition Leader
You have synchronizers in your transfer cases? If so then you have the same issue we Toyota guys have had forever in our manual gear boxes and finding GL4.

It's not that GL5 is too slippery (although that may be an issue with shifting). That's a separate issue and depends on the friction modifiers the particular oil has and I believe a question regardless of GL4 or GL5 spec. IOW you can have a GL4 that still doesn't work well with synchros I think.

The problem for me is the level of buffered sulfur and phosphorous used in the various specs. These additives are what react to make the extreme pressure, or EP function. Under pressure they create a sacrificial wear layer on the metals.

The issue is this layer is intended to peel off, which it does fine with steel. However on yellow metals the underlying metal is weaker than the layer so as it wears a few micron layer of brass or bronze is peeled off with it.

So it boils down to GL4 and GL5 and their intended uses. GL5 to the API is in every way better as a hypoid gear oil. But to achieve the higher EP rating the amount of phosphorous and its buffered sulfur are increased two fold and creates a thicker or stronger bonded layer.

This is great for differentials but since the spec doesn't care about yellow metals that means it just wears down your synchros faster. GL4 doesn't create nearly as strong of a sacrificial layer and the wear is greatly reduced on synchros.

FWIW I use Redline MT90 in my transmissions, which is a 75W90 GL4 fully synthetic. Redline makes other weights. Most of us find MT90 is the bee's knees, even helps make an old worn out transmission shift noticeably better.
Last edited:


Active member
Thanks for all the input guys. It’s so helpful to have a group that has been around these vehicles for so long. Going with GL-4 should be easy now I know it’s at NAPA.

Any idea what the “or higher” means in the FSM? I wonder if it is a mistake

Sent from my iPhone using Tapatalk