Does gear oil that meets API GL-5,also meets API GL-4?

GL-4 typically has about half the extreme pressure additives of a GL-5.

API classification subdivides all transmission oils into 6 classes:

* API GL-1, oils for light conditions. They consist of base oils without additives. Sometimes they contain small amounts of antioxidizing additives, corrosion inhibitors, depresants and antifoam additives. API GL-1 oils are designed for spiral-bevel, worm gears and manual transmissions without synchronizers in trucks and farming machines.

* API GL-2, oils for moderate conditions. They contain antiwear additives and are designed for worm gears. Recommended for proper lubrication of tractor and farming machine transmissions.

* API GL-3, oils for moderate conditions. Contain up to 2.7% antiwear additives. Designed for lubricating bevel and other gears of truck transmissions. Not recommended for hypoid gears.

* API GL-4, oils for various conditions - light to heavy. They contain up to 4.0% effective antiscuffing additives. Designed for bevel and hypoid gears which have small displacement of axes, the gearboxes of trucks, and axle units. Recommended for non-synchronized gearboxes of US trucks, tractors and buses and for main and other gears of all vehicles. These oils are basic for synchronized gearboxes, especially in Europe.

* API GL-5, oils for severe conditions. They contain up to 6.5% effective antiscuffing additives. The general application of oils in this class are for hypoid gears having significant displacement of axes. They are recommended as universal oils to all other units of mechanical transmission (except gearboxes). Oils in this class, which have special approval of vehicle manufacturers, can be used in synchronized manual gearboxes only. API GL-5 oils can be used in limited slip differentials if they correspond to the requirements of specification MIL-L-2105D or ZF TE-ML-05. In this case the designation of class will be another, for example API GL-5+ or API GL-5 LS.

* API GL-6, oils for very heavy conditions (high speeds of sliding and significant shock loadings). They contain up to 10% high performance antiscuffing additives. They are designed for hypoid gears with significant displacement of axes. Class API GL-6 is not applied any more as it is considered that class API GL-5 well enough meets the most severe requirements.

(source: WIKI)

Dear Tempest& EM
I heard that GL-5 gear oil is too aggressive to yellow metals in transmission which needs GL-4.
Usually we can think that higher specification covers lower, but I don't know if in gear oils you can use GL-5, where GL-4 is required?

Don't fully understand the chemistry but it is generally accepted that modern GL-5 is not corrosive to yellow or soft metals. Several people have reported poor shifting in manual transmissions with GL-5 gear oils since they are designed primarily for differentials.

If your application is a manual trans, I would look for a dedicated GL-4 fluid designed for that app. Amsoil and Redline are known to make such fluids. These fluids have special friction modifiers that help with shifting.

Simple yet complex question.
The answer lies in the application and the additive system used.
As Tempest says GL5 has more of the same additive than GL4, but more additive can effect friction properties in manual gear boxes( Shift feel etc) Look at the number of specs for car manual boxes.
Viscosity selection also to be considered.
80W-90 and 85W-140 are not true multigades(VI~100)
Problem not so severe in manual Truck boxes. Does your oil supplier use approved OEM additives to meet GL4/5 performance. Look for approvals in product data sheets not just suitability. Yellow metal can still be a problem with some GL4/GL5 oils.

Simple answer is no. Just because a oil meets GL-5 does not mean it also meats GL-4. Most do now day's but it's not a guarantee. The best thing to do is consult the data sheet of the lubricating oil in question. If it doesn't list that it meets GL-4 then it probably doesn't.