GL is short for " GEAR LUBRICANT"

API has the following GL classifications: for Gear Oils -

GL-1: Oil without EP additive.

GL-2: Usually contains fatty materials.

GL-3: Contains a mild EP additive.

GL-4: Equivalent to MIL-L-2105B and is usually satisfied by a 50% GL-5 additive level.

GL-5: Equivalent to MIL-PRF-2105E. Primary field service recommendation for Passenger cars and trucks worldwide.

GL-6: For severe service involving high offset hypoid gears. Often used to describe oils used in limited slip differentials. (Obsolete now)

MT-1: For non-synchronised manual transmissions in buses and trucks at a higher level than GL-4.

MIL-PRF-2105E: Designed by the US military, it takes conventional GL-5 and adds more demands to the specification. Most hypoid oils conform to this standard. Now superseded by SAE J2360 (2003).

The Difference between GL-4 and GL-5 Gear Oils

Many people are confused about API gear oil classifications. It is a common belief that API GL-5 oils can be used where API GL-4 requirements are specified. Although this is true for “gear oil”, it does not make API GL-5 gear oils satisfactory for all transmissions especially those that use synchromesh. They may meet the “gear oil” specifications, not the transmission oil specifications. API GL-4 and GL-5 categories do not mention or have anything to do with transmission synchronisers.

Originally, lead additives were used in gear oils and were very good at reducing wear in loaded gear sets. Lead though, is not very friendly to the environment and was phased out in favour of a Phosphorus and Sulphur combination. These gear oils used the Phosphorus and the Sulphur to attach to the gears and create a strong sacrificial layer to be worn off over the life of the fluid, thus protecting the gear from abrasive wear, high load and shock loads. Initially, the problem with this type of formulation was that the Sulphur used was active and caused corrosion of yellow and other soft metals used in transmissions and differentials. This happened because active sulphur reacted with some metals and metal alloys, especially those that used copper as the alloying element to form metal sulphides that in turn caused corrosion of the metal.

A generation ago deactivated or buffered sulfur was developed that could react with the phosphorous and create a protective and sacrificial layer in conditions created inside gear boxes. This formulation was not corrosive to brass, copper or other metal alloys used in transmissions. Today this is widely used in automotive transmission and gear oils.

Although de-activated sulphur may be used in a gear oil and may be safe for yellow metals such as brass and copper, it does not address the question of whether a GL-5 product can be used in all transmissions and especially those with synchromesh. A GL-5 grade oil has a very high rating for EP (Extreme Pressure) protection. EP gear oils contain additives that prevent metal surfaces from cold welding under the extreme pressure conditions found in situations where boundary lubrication prevails. At the high local temperatures associated with metal-to-metal contact, an EP additive combines chemically with the metal to form a surface film that is ductile enough to prevent the welding of opposing surfaces and prevent scuffing or scoring that is destructive to sliding surfaces under high loads.

In normal operation, the sulfur/phosphorous additive forms a black sacrificial coating on the gears and anything it touches with a little pressure and temperature. As the gears turn, instead of wearing, the sacrificial coating of additives is peeled off or worn off. This is normal and acceptable in all steel gears. But when one or more of the surfaces is brass or another soft metal, the sacrificial coating is stronger than the base metal, and instead of just peeling off, it takes with it a few microns of the softer metal.

An API GL-4 gear oil of any given viscosity has about ½ of the level of sulfur/phosphorous additive that would be in the API GL-5 product, so the bond with the metal surface inside the transmission is not as strong, and therefore can be peeled off without peeling a layer of soft metal. This means that the GL-4 product provides a little less extreme pressure protection than a GL-5 oil but less wear on synchromesh components of a transmission. When a GL-5 oil is used in a transmission with synchromesh it can create up to 4 times the amount of copper in a used oil analysis as that of a GL-4 product. Synchronisers will eventually wear to the point where they become ineffective.

When used in a differential, there is a 30% increase in the torque load compared with a transmission, so in this case, the extra EP protection is required, which is where an API GL-5 oil is best suited.

Although API GL-3 oils are considered obsolete as they have less protection than API GL-4 oils, some transmission manufacturers may still specify an API GL-3 gear oil. There are many formulations of GL-3 oils in the market, some with sulfur/phosphorous additive and some with zinc/phosphorous. Many diesel engine oils classify as a GL-3 or GL-4 in gear protection.

HONDA MTF-94/ROVER MTF-94:

Describes a GL-4 type 10W-30/75W-80 oil that is semi synthetic for long drain and good low temperature shift feel.

MAN 341:

API GL-4 type. There are many variance of this specification.

MAN 342:

API GL-5 type

CATERPILLAR TO-4:

The SAE 30, 50 & 60 versions can be used in manual transmissions and some final drive units.

CATERPILLAR FD-1:

Describes a fluid used in differentials of heavy duty Caterpillar equipment where additional wear protection is needed. Cannot be used in wet-brake applications. Usually a SAE 60 grade or multigrade synthetic.

MB 235.5:

Heavy duty API GL-4.

MB 235.0/235.6:

Heavy duty API GL-5 type oils for long drains.

MB 235.10:

Light duty, synthetic performance 75W-80 for MercedesBenz Sinter Synchromesh transmissions.

MACK GO-J:

Designed to deliver 250,000km oil drain intervals. More severe than API GL-5. GO-J/S is the synthetic version.

MACK TO-A PLUS:

Specialised manual transmission fluid with long life.

VOLVO 1273.07:

SAE 30 type (SAE 80) oil based on GL-4.

VOLVO 1273.10:

API GL-5, SAE 80, 90.

VOLVO 1273.12:

SAE 50 (SAE 90) type usually satisfied by TO-4 type oils.

ZF:

• TE ML-01: Non-synchro, heavy duty manual truck transmissions.
• TE ML-02: Manual and automatic transmissions for trucks, buses and light commercials. Various sub-groups.
• TE ML-03: Transmissions in off road vehicles.
• TE ML-04: Marine transmissions, SAE 30/40 engine oils.
• TE ML-05: Axles in off road vehicles. Various sub groups for different grades and types.
• TE ML-06: Tractor transmissions and hydraulics.
• TE ML-07: Hydrostatic and mechanical drives and electric drive systems.
• TE ML-11: Manual and automatic transmissions in cars.
• TE ML-12: Axles for cars, commercial vehicles and buses. Various sub-groups.
• TE ML-14: Powershift transmissions, Ecomat type, for trucks and buses.

BTR:

5M-42, 5M-31, 5M–36, 5M-41, 5M-50, 5M-48.

Holden:

HN1855, HN1820, HN1046, HN1070, HN1181, HN 386, HN1561, HN1187, HN 2013, HN2040

ROCKWELL:

O-76A, O-76B, O-76N, O-76D.

EATON FULLER:

PS 164 (Rev. 7) (For Roadranger transmissions).