Decoding Motul Gear Oil Specs: API GL-4 vs GL-5 and Viscosity Explained

The specification codes printed on Motul gear oil bottles—such as API GL-4, API GL-5, and SAE viscosity ratings like 75W-90—serve as a precise guide to the fluid’s chemical makeup and physical behavior under stress. Understanding these codes is essential for maintaining the health of your vehicle’s manual transmission, differential, or transfer case. Using the wrong specification can lead to accelerated wear, poor shifting quality, or even catastrophic component failure.

To make an informed purchase, vehicle owners must decode these labels and match them directly to the requirements set by their vehicle’s manufacturer. This guide breaks down the critical differences between API service classifications and viscosity grades so you can select the correct fluid for your drivetrain.

What API GL-4 and GL-5 Mean for Your Transmission

The American Petroleum Institute (API) gear lubricant classifications indicate the level of extreme pressure (EP) protection the oil provides. The primary difference between API GL-4 and API GL-5 lies in the concentration of these EP additives, which are typically sulfur-phosphorus compounds designed to protect gears under heavy loads.

Products mentioned in this guide

Motul 🇸🇬 MOTUL Motylgear 75W90 Technosynthese Gear Oil Manual Transmission Differential [1L] S$39.00 View product →

Motul 🇸🇬 MOTUL Gear 300 75W90 Fully Synthetic Manual Transmission & Gear Oil [1L] S$39.00 View product →

Motul 🇸🇬 MOTUL Gear 300 LS 75W90 Fully Synthetic LSD Differential Gear Oil [1L] S$39.00 View product →

Motul 🇸🇬 MOTUL Gear FF-LSD Type 2 75W90 Fully Synthetic Gear Oil Limited Slip Differential [1L] S$39.00 View product →

Motul 🇸🇬 MOTUL High-Torque DCTF Dual Clutch Transmission Fluid Synthetic Gearbox Oil [1L] S$39.00 View product →

Prices are valid at publication and may change. Please refer to the product page for the latest price.

API GL-4 gear oils are formulated for moderate-to-severe operating conditions and are commonly specified for manual transmissions and transaxles. They contain a balanced level of EP additives that protect gears without harming sensitive internal components.

API GL-5 gear oils are designed for highly loaded gears, such as hypoid differentials, where sliding friction and extreme pressures are common. To handle these intense forces, GL-5 oils contain roughly double the concentration of sulfur-phosphorus EP additives compared to GL-4 oils.

While the higher additive concentration in GL-5 provides superior protection for heavy-duty gears, it can be highly detrimental to manual transmissions containing yellow metals. Many manual gearboxes use brass or bronze synchronizers to match gear speeds during shifting. Under high operating temperatures and pressures, the active sulfur in GL-5 oil can chemically react with these copper-alloy components, bonding to them and peeling away microscopic layers of metal over time.

Because of this chemical reaction, a higher API number does not automatically translate to better performance or superior protection for every application. Selecting gear oil is not a matter of upgrading to a higher number; it is a matter of strict compatibility. You must always consult your vehicle manufacturer’s manual to determine whether your system requires GL-4 or GL-5.

How to Read SAE Viscosity Grades on Gear Oil

The Society of Automotive Engineers (SAE) viscosity grade (such as 75W-90 or 80W-90) describes how the gear oil flows at both low and high temperatures. These multi-grade oils are engineered to provide reliable lubrication across a wide range of operating conditions.

The number preceding the “W” stands for “Winter” and indicates the oil’s low-temperature fluidity. A lower number, such as 75W, means the oil remains more fluid in cold temperatures, allowing it to circulate quickly during cold starts and reducing initial wear on drivetrain components.

The number following the “W” represents the oil’s kinematic viscosity at high operating temperatures, typically measured at 100 degrees Celsius. A higher number, such as 90 or 140, indicates a thicker oil film that provides robust protection under heavy loads and high-temperature driving conditions.

Multi-grade gear oils use specialized base oils and viscosity index improvers to adapt to these temperature swings. This adaptability ensures that the fluid is thin enough to flow easily when the vehicle is first started, yet thick enough to maintain a protective barrier between gear teeth once the drivetrain reaches its normal operating temperature.

It is a common misconception that a thicker gear oil always provides better protection. While a higher viscosity grade offers a thicker protective film, it also increases internal fluid drag, which can raise operating temperatures, reduce fuel efficiency, and cause stiff or difficult gear shifting. Viscosity selection must always align with the original equipment manufacturer (OEM) specifications to ensure proper lubrication and system efficiency.

Step-by-Step Guide to Matching Gear Oil to Your Vehicle

To ensure the Motul gear oil you select is fully compatible with your vehicle’s drivetrain, follow this systematic verification process before purchasing or performing any maintenance.

First, locate the exact fluid specifications required for your vehicle. This information is typically found in the “Specifications,” “Maintenance,” or “Technical Data” section of your owner’s manual. Note both the required API service classification (such as GL-4 or GL-5) and the recommended SAE viscosity grade (such as 75W-90).

Second, cross-reference these requirements with the product label on the Motul gear oil bottle. Motul clearly prints the API classifications, viscosity grades, and specific manufacturer approvals on the front and back labels. Ensure that the product matches both the API rating and the viscosity grade specified in your manual.

Third, identify any special drivetrain configurations that require specialized fluids. For example, vehicles equipped with limited-slip differentials (LSD) often require gear oils containing specific friction modifiers to prevent clutch chatter and ensure smooth operation. Look for Motul products labeled specifically for LSD applications, or check if the product description explicitly states that it contains the necessary additives.

Finally, observe strict safety boundaries when selecting and changing your gear oil. Never substitute a GL-5 fluid into a manual transmission requiring GL-4 unless the vehicle manufacturer explicitly permits it, or the Motul product label guarantees that the fluid is dual-rated (GL-4/GL-5) and formulated to be completely safe for yellow metals.

When performing a fluid change, always follow the original-vehicle instructions and safety protocols. Ensure the vehicle is securely supported on jack stands or a professional hoist, use appropriate personal protective equipment, and dispose of used fluids responsibly. If you lack the proper tools, experience, or confidence to perform this maintenance safely, seek qualified professional installation at a licensed service center.

Frequently Asked Questions (FAQ)

Can You Mix API GL-4 and GL-5 Gear Oils?

No, you should not mix API GL-4 and API GL-5 gear oils. Blending these two different classifications alters the precise chemical balance and additive concentrations of the fluid. This dilution can compromise the extreme pressure protection required for hypoid gears or introduce excessive sulfur levels that can damage brass synchronizers in manual transmissions. To ensure optimal protection and performance, always perform a complete drain of the old fluid before refilling the system with the correct specification.

Does a Higher Viscosity Grade Mean Better Protection?

A higher viscosity grade does not universally mean better protection. While a thicker oil film can support heavier loads, excessively high viscosity can restrict fluid flow, increase operating temperatures due to internal friction, and cause sluggish gear engagement. Modern transmissions and differentials are engineered with precise clearances that require specific viscosity profiles to function correctly. Always adhere to the OEM-recommended viscosity grade to maintain the correct balance of protection, cooling, and efficiency.