Short answer
API GL-4 is an American Petroleum Institute gear lubricant designation for spiral-bevel axles under moderate to severe conditions and hypoid axles under moderate conditions, and it is also used in selected manual transmissions and transaxles. API now lists GL-4 as not in current use because the test equipment for verifying it is no longer available, but the term is still widely used to describe gear oils.
What API 1560 says about GL-4
API’s gear oil designations are published in API 1560. The ninth edition, from November 2023, describes GL-4 as lubricants for axles with spiral bevel gears operating under moderate to severe speed and load, or hypoid gears under moderate speed and load. It adds that GL-4 oils may be used in selected manual transmission and transaxle applications where API MT-1 lubricants are unsuitable.
For limited-slip axles, API notes that frictional requirements are normally defined by the axle manufacturer, so GL-4 alone does not cover them.
Why GL-4 is “not in current use”
GL-4 was originally verified with tests listed in an old ASTM publication, and most of that test hardware went out of production long ago. API’s Lubricants Group was already weighing whether to declare GL-4 a designation no longer in use around 2010, and API 1560 now places it in that group alongside GL-1, GL-2, GL-3, GL-6 and MT-1.
API also notes that GL-4 is still used commercially to describe lubricants. Its advice is for end users to ask suppliers for supporting documentation on previously tested lubricants.
The “half the additive of GL-5” myth
GL-4 is often described as a gear oil with about half the extreme-pressure additive level of GL-5. Trade press coverage of API’s 2010 discussions noted that this 50 percent treat-rate approach is not documented in any API, SAE or ASTM publication, and that a GL-4 claim based only on that shortcut needs additional support.
In practice, that means a GL-4 label is best treated as a description that should be backed by the oil marketer’s data, and by the transmission maker’s own fluid specification where one exists.
Where GL-4 is still specified
Many manual transmissions and transaxles, especially older passenger-car and light-truck designs with synchronizers, list GL-4 in the owner’s manual. Some current transmissions call for a dedicated manual transmission fluid with its own viscosity and friction requirements instead. Read the manual for the exact designation and SAE viscosity grade before choosing a fluid.
Frequently asked questions
Does API still license GL-4 gear oils?
API does not run a licensing program for gear oil designations like its engine oil donut, and API 1560 lists GL-4 as not in current use. The term still appears on labels, so ask for supporting data.
Can I use GL-5 where GL-4 is specified?
Not automatically. Some manual transmissions are sensitive to the additive chemistry used in GL-5 oils. Use what the transmission maker specifies, or a fluid that lists both designations and the maker’s approval.
Is GL-4 a viscosity grade?
No. GL-4 describes performance. Viscosity is a separate SAE J306 grade such as 75W-90.
General maintenance information. Always follow your vehicle or equipment manufacturer’s requirements and the current product data. Questions? Email [email protected].