I got curious about GL-4 and GL-5 since O'Reilly didn't carry GL-4 except for 140 weight, and I ran across this paper. It tells exactly why we don't want GL-5 in our xfer boxes. We might want this on the FAQ page.


The difference between gl4 and gl5 gear oils

"Does that make them satisfactory for synchromesh or synchronized transmissions?"

The article addresses the issue with using GL5 in a synchronized gear box.

FYI On a GEN1 and GEN2 there are no synchronizers in the transfer case to have a wear problem with. There are bronze thrust washers. However, the loading and heat level are not sufficient to cause any problems. I have never had the transfer cases apart on the later models.

The transmission is a different story and GL5 can reduce synchro life.

Better safe than sorry. The fluid specifications list GL-4 for the xfer box and GL-5 for the diffs, no 'or' notes that I've seen. Since the 4wd is shift on the fly, up to 65 mph, there must be some sort of synchronization going on. It's true that GL-5 is a better extreme pressure gear lube, as long as you have only steel components.

My main thrust was dissemination of information, and to warn people about the lubes that say they meet both specs, they do as far as extreme pressure goes, but are not safe for yellow metal.

The original manual lists GL-4 or higher for both the transfer case and the transmission. There was a TSB in 1993 that corrects that callout for the transmission to Gl-4 only.

Again, for the transfer case its a non issue. I have been running Gl-5 in my transfer case since 1994 without any problem. That includes inspecting parts when I installed lower ratio low range transfer case gears about 12 years ago.

I'm going to go out on a limb here and say that If I haven't had a problem in 22 years, its probably safe.

You do bring up a good point that GL-5 is a problem in some applications.

"Modern EP gear oils contain thermally stable additives that promote system cleanliness and do not corrode yellow metals under moderate operating conditions. The goal in formulating top-tier EP gear oils is to develop additive chemistries that will carry high loads under boundary conditions and protect mating surfaces from wear while minimizing corrosiveness to yellow metals and keeping steel gear components clean."

Gear Oil EP additives

I have been running Redline 75-90 Gear GL-5 lube as well as Mobil 1. Same Redline in the front LSD and the rear Torsen. That simplifies what I need to keep on the shelf in the shop.


GL-4 GL-5 TSB reference

I accidentally put GL-5 into a '97 Montero Sport transfer case several years ago. When I drained the gear oil it looked like liquid gold. Needless to say that was the end of that transfer case.

Edward

We should have a fluid specification sticky

Update... Until I can confirm I should have said no synchros in a GEN 1.

If I'm not mistaken, the Montero Sport 4WD '97-04 and the AWD '00-04 use the same transfer cases as the Montero Gen 2 and forward? I do know that all the Sport t-cases have syncros so I presume everything post-97 (at least) in the Montero has syncros.

I apologize for asking a stupid question, but what is the generational year boundary for the Montero?

Edward

I understand that people are thinking that it's a corrosion problem, which it's not. The sulphur is buffered so as not to be corrosive. The issue is that the sacrificial boundary layer that forms with GL-5 (GL-4 forms one too, just not as strong) has a surface bond strength that is stronger than the brass itself, thus it is a physical erosion problem. The difference is irrelevant except if you have someone telling you its OK because they have had a synchro sitting in a glass jar of GL-5 for years and it's still ok, so the stuff is good to use. I wasn't aware of the TSB, makes you think about all of the comments here to the effect of "I'm not going to try to outguess the Mitsubishi engineers."

Which is why driving mine for two months left me with a catch pan full of brass particles and tiny flakes.

Edward