Gear oil: GL-4 or GL-5, 75W-90 or 80W-90?

So your car’s service book specifies “API GL-4, SAE 75W-90” for the manual gearbox. On the shelf, side by side, stand cans marked GL-4, GL-5, “GL-4/GL-5”, “GL-4+” and even something called MT-1, and the salesman advises: “Take the GL-5, it’s higher, it covers everything.” Sound familiar? In fact GL-4 and GL-5 are not two rungs on a quality ladder but oils for different components, and the “higher” GL-5 can do harm in a gearbox that needs GL-4.

The essentials were already covered in the article on fluids for transmissions: GL-5 is designed for the hypoid gears of axles, GL-4 is specified for certain manual gearboxes, and the SAE J306 viscosity standard gained new, thinner grades in its 2019 edition. Here we look more closely at what lies behind each marking on the can.

What API itself says: the range from GL-1 to MT-1

Gear oil categories are set out in document 1560 of the American Petroleum Institute (API). In its ninth edition (November 2023) the range looks like this:

Two things here surprise many people. First, by API’s definition GL-4 is primarily a category for axles, and it goes into gearboxes where the manufacturer has decided it should. Second, API recognises only GL-5 as current and has moved the rest, GL-4 and MT-1 included, to “not in use”: carmakers changed their requirements, or the test equipment ceased to exist. About GL-4 API is blunt: the marking is still used on cans, but there is nothing left to test it on, so buyers are advised to ask the supplier for documentation of earlier tests.

Where, then, does GL-4 on new cans come from? A lubricants trade magazine described a common method back in 2010: take an additive package that has passed the GL-5 tests and use it at half the treat rate. As the same magazine wrote, no API, SAE or ASTM document describes this method. So “GL-4” on a new can is, as a rule, the oil maker’s word: today there is nothing to check it on by an API method.

Why GL-5 does not replace GL-4: synchronisers and sulphur

In an axle’s hypoid gear set the teeth do not just roll, they also slide hard against each other under enormous pressure. To stop the metal scuffing, sulphur-phosphorus EP additives are put in the oil: heated in the contact zone, they react with the steel surface and form a protective film. For an axle, that is exactly what is needed.

In a synchromesh gearbox, however, the synchroniser rings may be brass, sintered bronze or molybdenum-coated, and the oil needs a certain amount of friction there so that the synchroniser can match the speeds of the gears. According to the trade magazine Machinery Lubrication, GL-4 oils mostly contain half as much sulphur-phosphorus additive as GL-5 oils, and an excess of these additives can attack soft “yellow” metal when it is hot. On top of that, an oil formulated for an axle often lacks the friction properties a synchroniser needs. The result is prematurely worn synchronisers, and with them a crunch when changing gear. It is a bit like oven cleaner: it takes off burnt-on grease splendidly, but nobody cleans the silver spoons with it.

Hence a few rules for reading a can.

1. “GL-4/GL-5” on one can. This is the oil maker declaring that the oil suits both a gearbox and an axle. Of the oils in our catalogue that carry GL categories, roughly one in five claims both. But it is the oil maker’s claim: whether it suits your particular gearbox is decided by the entry in the vehicle’s documentation, not by a dual marking.

2. “GL-4+” (GL-4 Plus). API 1560 has no such category at all. What the “plus” means is decided only by whoever printed it.

3. MT-1. According to API, this category is for non-synchronised gearboxes of trucks and buses, and API specifically points out that it does not cover the requirements of synchromesh gearboxes in passenger cars. So MT-1 on a can says nothing about whether the oil suits a synchromesh car gearbox.

Axle, gearbox, transaxle: what goes where

1. A drive axle with hypoid gears (rear-wheel-drive cars, off-roaders, trucks) is GL-5 territory. If the axle has a limited-slip differential (LSD), API points out that the friction requirements here are set by the axle manufacturer. Oils marked LS contain friction modifiers so that the differential does not chatter or judder, and where the manufacturer does not call for them they are simply superfluous: such additives, as a rule, tolerate heat less well.

2. A transaxle is a gearbox and differential in one housing, as in most front-wheel-drive cars. As a rule one oil lubricates both the gearbox gears and the final drive, so the requirement comes from the maker of that particular unit. Sometimes it is GL-4, sometimes the manufacturer’s own code, and some manufacturers even specify engine oil for their gearboxes.

3. Trucks and buses. Here axles often call for not just GL-5 but SAE J2360. This specification grew out of military ones: in 1995 MIL-PRF-2105E replaced MIL-L-2105D, and in 2005 it was cancelled and replaced by SAE J2360 (SAE had adopted that standard back in 1998). According to API, an oil approved to J2360 meets GL-5 and goes beyond it (seal compatibility, gear cleanliness after oxidation), and the list of approved oils, according to the SAE standard itself, is kept by the Lubricant Review Institute. So “meets MIL-L-2105D” on a can is a claim of conformity to a specification that was replaced back in 1995.

And the codes of gearbox and axle makers — ZF TE-ML, MAN 341 and 342, Scania STO, VW G 052 … — carry more weight than any API category; what they mean is explained in the article on ZF TE-ML approvals.

75W-90 or 80W-90: the SAE J306 table

The viscosity of gear oils is defined by the SAE J306 standard, and its numbers have nothing in common with the SAE J300 grades for engine oils. Here too the letter W means a “winter” grade, but it is tested differently: for each W grade the standard sets the highest temperature at which the oil’s viscosity reaches 150,000 mPa·s. The limit was not chosen at random: with thicker oil the pinion bearings failed in tests, and at that viscosity the oil is like a semi-fluid grease that barely pours.

The table below follows the February 2019 edition of SAE J306; in February 2025 SAE issued a new edition of the standard.

(In the standard, the upper limit of each range is not part of the grade.)

Now look at the title. In a warmed-up unit 75W-90 and 80W-90 belong to the same grade: both are SAE 90, that is, 13.5-18.5 mm²/s at 100 °C. The difference is in the cold: 75W stays within the limit down to −40 °C, 80W down to −26 °C. Use 80W-90 instead of 75W-90 only if the gearbox manufacturer allows both grades. And remember: −26 °C for 80W is not a temperature at which it is comfortable to use, but the limit at which the oil barely flows any more; 75W-90 is thinner in frosty weather, so a cold gearbox shifts more easily with it. 85W-140, on the other hand, reaches its limit as early as −12 °C, so in winter it goes in only where the manufacturer expressly permits it.

Do not smile — the “90” on a gear oil is thinner than it looks. Going by the number, it is twice as thick as the “40” on an engine oil, but at 100 °C grade 90 (13.5-18.5 mm²/s) overlaps the top of engine-oil SAE 40 and the bottom of SAE 50 (see the SAE J300 viscosity table). These are two different scales. And one more thing: the new grades 65, 70 and 75, without the letter W, appeared in 2019 for modern gearboxes designed for a very thin oil. A “75” oil and a “75W-90” oil are not the same thing at all.

What does this mean for the ordinary car owner?

1. The carmaker’s requirement comes first. First the code of the vehicle or unit manufacturer, then the API category, then the viscosity, and all of it from the service book or owner’s manual.

2. The words on the can are the oil maker’s claim. “Meets”, “GL-4/GL-5”, “GL-4+”, “MT-1” guarantee nothing in themselves; how to tell an approval from a claim is explained in the article on how to check an oil approval.

3. Not “higher”, but the same. Put GL-5 in a gearbox that calls for GL-4 only if the gearbox manufacturer expressly allows it.

4. Changing the oil. Manufacturers class many gearboxes as maintenance-free; in that case you check the level and top up with the same oil (more in the article on changing gear oil). On a used car with an unknown history it is worth having the level and condition of the oil checked at a garage that knows that particular gearbox; change it only if necessary, following the manufacturer’s procedure and with the oil it specifies.

I will say it again: gear oil is chosen just like engine oil, by the carmaker’s requirement and not by the bigger number after GL. Repairing a gearbox or an axle costs incomparably more than the right can, so leave “it covers everything” to the salesman.