Community Member Credit: Frank / motorvate.ca

For the average automobile, there are two types of API Service for gear oil:

• GL-4
• GL-5

The “API Service” has nothing to do with viscosity. Viscosity is basically the “thickness” of the oil at winter (75W) temperatures, or summer (80) temperatures. Some typical examples as 75W80, and 80W90. Consult your owner’s manual for what is best for your vehicle, in your climate, with your driving conditions. An “API Service” is a specification that defines flash point, viscosity, color, pour point, the amount of extreme pressure additives and others.

Generally GL4 is for transmissions from European and Japanese auto manufactures. GL5 is usually for transmissions and differentials from North American auto manufactures.

As with most specifications, the newer the better. So GL-5 is better than GL-4. GL5 has about twice the extreme pressure (EP) additives of GL-4. Due to these additives the transmission must be designed for GL-5.

If you buy GL-5 gear oil, and put it in your GL-4 transmission you are not doing your transmission a favor. In some cases the GL-5 gear oil can be “too slippery” and the synchronizers in the transmission will slightly “crunch”, as they are not rotating at the same speed. GL-5 also has twice the extreme pressure additives of GL-4, and these additives can corrode certain “yellow” metal synchronizers. Think about that!

With this in mind, if you have any transmission work done, insist on GL-4, if your owner’s manual calls for it. My 95 Maxima calls for GL-4, and I had a bearing failure (not related to GL-4 / GL-5, but an excessive preload) When I asked them what kind of oil did they put in the transmission, they said “Esso 75W-90.” I then asked to see the container that it was in, and right on the front it said GL-5. I asked them, “Is it OK to use GL-5 in a car that is designed for GL-4?”, and their answer “Yes, it is one better” They also said that the gear oil they use is approved by Nissan. I called Nissan HO, and asked them if GL-5 can be used, and the tech on the line told me, “GL-4 should be used.” Hmmm.

After paying $1200 to rebuild my transmission, I now had buy GL-4 gear oil and change it myself. Not a big deal, but to my surprise no one, and I mean no one sells GL-4. I tried eight local auto stores, and they all sell GL-5. No GL-4! To make a long story short I found some GL-4 in Rochester, NY and replaced the GL-5. The shifting improvement was immediate. With the GL-5 when I would shift I could feel the syncronizers hitting just before the car went into gear. With the GL-4 it was like just shifting a gear, with the usual amount of “snick” (aka notchiness)

The following is a technical note from Pennzoil, P-21-B which talks about GL-4 & GL-5
(The below text is found on page 2)

GL-5 GEAR OILS ARE NOT INTERCHANGEABLE WITH GL-4 GEAR OILS.

• The extreme pressure (EP) additives in most gear oils contain sulfur-phosphorus compounds that can be corrosive to certain “yellow” metal components such as copper and brass synchronizers.
• Always follow your owner’s manual for the correct lubrication recommendation.

Simple rule: RTFM (read the friggin manual) and follow it.

The manufacture knows what is best, the dealer may not.