Gl4 vs Gl5 Gear Oil: Which One Should You Buy?

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Below: what the GL rating means in these listings, how the GL-4 listings differ, and a checklist for matching a listing to a transmission.

Gl4 vs Gl5 Gear Oil: What the Rating Means

In these listings, the GL rating appears alongside the application, not as a quality ladder. The Red Line listing recommends its oil for GL-1, GL-3 and GL-4 applications, and for places where most special synchromesh fluids are specified. The CRC listing states suitability for standard transmissions and differentials requiring API GL-3 and GL-4 oils. The Liqui Moly listing describes its oil for transmissions and axle drives where an API GL 4 gear oil is required. Read together, the listings treat the rating as a maker specification to match, and the viscosity class as another specification to match, with the Liqui Moly listing warning to keep to the viscosity class stipulated by the transmission manufacturer.

This guide compares the GL-4 listings in the gear section below against the GL-5 contrast the Red Line listing describes, staying with what the listings state and adding no guess.

Gear Picks From the Listings

Red Line MT-85, 1 Quart

The Red Line listing presents its MT-85 oil as a GL-4 gear oil in a 1 Quart bottle, with a viscosity class of 75W85. The listing recommends it for GL-1, GL-3 and GL-4 applications, and for places where most special synchromesh fluids are specified, with gear protection, longer synchro life, and quicker shifts from balanced friction control. The listing also says the oil is safe for brass synchros, because its GL-4 oil lacks the reactive sulfurs found in most GL-5 oils that cause damage, and it names synchro manual transmission applications for Nissan, Mitsubishi Evolution, Hyundai, Kia and Quaife. Note a contradiction in the listing: the spec table lists a viscosity of 5w-30, while the title lists 75W85, and this guide follows the title and the bullets.

Liqui Moly Gear Oil, 1 L

The Liqui Moly listing presents a mineral transmission oil in a 1 L bottle, with a viscosity class of 85W-90, for transmissions, particularly manual transmissions, and axle drives with normal to high loading where an API GL 4 gear oil is required. The listing claims a stable lubricating film, wear reduction, extremely good lubrication properties and outstanding corrosion protection. The listing also says to keep to the viscosity class stipulated by the transmission manufacturer, that the oil is miscible with all branded gear oils, and that it is fully effective only when used alone. The listing pairs those points: mixing is possible, but full effectiveness is tied to using the oil alone.

CRC Sta-Lube Gear Oil, 1 Gallon

The CRC listing presents a multi-purpose GL-4 gear oil in a 1 Gallon bottle, with a viscosity class of 85W90. The listing claims low pour points and high temperature properties for lubrication over a wide temperature range, with anti-rust and anti-wear EP additives for corrosion protection in drives and hypoid gears. The listing states the formula is not corrosive to copper, bronze or other non-ferrous alloy bearings and bushings, and that it is safe for yellow metals. For applications, the listing states suitability for auto, truck and bus standard transmissions, and differentials requiring API GL-3 and GL-4 oils, which makes this listing the clearest differential example in the set.

Red Line MT-85, 4 Pack

The Red Line 4 Pack listing presents, in its title and bullets, the same MT-85 oil as the quart listing above, with a viscosity class of 75W85, in a 4 Pack. The bullets repeat the same claims: gear protection, longer synchro life, quicker shifts, balanced friction control, and safety for brass synchros, with the same recommendation for GL-1, GL-3 and GL-4 applications and synchromesh fluids. The listing names the same synchro manual transmission applications for Nissan, Mitsubishi Evolution, Hyundai, Kia and Quaife. The difference in the title and bullets here is pack size alone, so a reader who already matches this oil should compare the pack size with the job, rather than treating the 4 Pack as a different oil.

How to Choose a Listing for Your Transmission

Start with the maker specification, not with the rating number. Check whether the transmission maker specifies GL-4, and check the viscosity class the maker stipulates, since the listings here state 75W85 and 85W90, with the Liqui Moly listing writing the second class as 85W-90. Next, check the application: Red Line points to synchro manual transmissions, Liqui Moly points to manual transmissions and axle drives, and CRC points to standard transmissions and differentials requiring GL-3 and GL-4. For brass synchros in a transmission that specifies GL-4, stay with the GL-4 specification, because the Red Line listing ties damage to the reactive sulfurs found in most GL-5 oils. Finally, check pack size against the job: the listings offer a 1 Quart bottle, a 1 L bottle, a 1 Gallon bottle, and a 4 Pack, and the right size is the match for the job without leaving a reader to guess at a volume the listings do not state.

FAQ: garage tools

Can I use GL-5 oil where GL-4 is specified?

Match the GL rating the maker specifies instead of substituting a GL-5 listing where GL-4 is specified, especially for brass synchros, because the Red Line listing says most GL-5 oils contain reactive sulfurs that cause damage. Match the GL rating the maker specifies, and match the viscosity class as well.

Is GL-5 better because the number is higher?

No. The rating is a specification to match, not a ladder, and Red Line recommends its oil for GL-1, GL-3 and GL-4 applications, while CRC lists GL-4 for differentials requiring GL-3 and GL-4. A higher number in the rating does not make an oil right for every transmission.

What viscosity class should I pick?

Pick the viscosity class stipulated by the transmission manufacturer, as the Liqui Moly listing warns. The listings here state 75W85 and 85W90, with the Liqui Moly listing writing the second class as 85W-90, so the choice is the class the maker specifies, not the class that sounds heavier or lighter.

Can GL-4 oil go in a differential?

Yes, where the differential requires GL-3 and GL-4 oils, as the CRC listing states. The Liqui Moly listing separately covers axle drives where an API GL 4 oil is required, which is an axle-drive specification rather than a differential permission. That permission is tied to the specification, so a differential that specifies a different rating needs a different listing.

What does the Red Line listing say about GL-5 oils?

The Red Line listing says its GL-4 oil lacks the reactive sulfurs found in most GL-5 oils that cause damage, and it recommends its oil for GL-1, GL-3 and GL-4 applications. Match the GL rating the transmission maker specifies before choosing a listing.

Related: best gear oil GL-5, best gear oil for trucks and best heavy duty gear oil.