Red Line MT-90 75W90 GL-4 Gear Oil – 946ml / 3.78L • 50304 / 50305

Red Line MT-90 75W90 GL-4 Gear Oil - 946ml / 3.78L • 50304 / 50305

High-performance Red Line synthetic gear oil for suitable synchronised manual transmissions and transaxles requiring SAE 75W90 API GL-4, GL-3, GL-1 or another specifically supported equivalent specification.

This is a combined listing allowing you to select either a 946ml / 1 US quart bottle (50304) or a 3.78L / 1 US gallon container (50305).

Red Line MT-90 is a full-synthetic ester-based manual transmission lubricant formulated specifically for synchronised gearboxes. Its shear-stable formulation provides consistent viscosity across a wide operating-temperature range while helping reduce gear noise, improve shift quality and maintain strong gear and synchroniser protection.

Unlike many highly slippery synthetic gear lubricants, MT-90 is formulated with an appropriate coefficient of friction for synchroniser engagement. This helps the synchronisers operate correctly while still providing the high film strength required to protect gears, bearings and other transmission components.

• Product type: Full-synthetic manual transmission gear oil
• Product range: MT-90
• Brand: Red Line Synthetic Oil
• Available sizes: 946ml / 1 US quart or 3.78L / 1 US gallon
• 946ml product reference: 50304
• 3.78L product reference: 50305
• SAE gear-oil viscosity: 75W90
• API service class: GL-4
• Also suitable for appropriate GL-3 / GL-1 applications
• Typical application: Synchronised manual transmissions and transaxles
• Viscosity at 100°C: approximately 15.5 cSt
• Viscosity at 40°C: approximately 82 cSt
• Viscosity index: approximately 200
• Brookfield viscosity at -40°C: approximately 320 Poise
• Pour point: approximately -45°C
• Base oil technology: Full-synthetic ester-based formulation
• Performance benefits: Improved shift quality, strong synchroniser protection, shear stability and high gear-tooth film strength

Mazda compatibility: Red Line specifically lists Mazda applications including Mazda MES 115A, Mazda 0000-77-114-QT and Mazda L.L. Manual Gear Oil IS. MazSpec currently lists MT-90 within its MX-5 Mk1/NA, Mk2/NB and Mk3/NC transmission ranges.

Fitment advice: Use only in manual transmissions or transaxles requiring an appropriate GL-4-type lubricant. Always match the transmission manufacturer's required viscosity and performance specification rather than selecting the oil solely by vehicle model.

Important differential note: MT-90 is a manual transmission / transaxle lubricant and should not be treated as a general-purpose differential oil. It is not intended for conventional hypoid differentials requiring an API GL-5 lubricant.

Which size should I choose? Select the 946ml bottle for smaller quantities, topping-up or where approximately one quart is required. Choose the 3.78L gallon container where a larger quantity is needed for servicing one or more transmissions. Always check the required gearbox capacity before ordering.

Other items to consider: When servicing a manual gearbox, you may also want to consider replacement drain and fill plug sealing washers, gearbox seals, gear-lever bushes and related transmission service components where required.

Useful tools / consumables: Fluid transfer pump or filling bottle, drain pan, suitable sockets or spanners, torque wrench, protective gloves and workshop towels.

Difficulty guide: DIY Difficulty: Moderate. Gearbox oil replacement is generally straightforward where the drain and fill plugs are accessible, but the correct lubricant specification, fill level and plug tightening torque must be followed.

Why Red Line? Red Line Synthetic Oil is known for high-performance lubricants, fluids and additives developed for demanding road, track and motorsport use. Red Line products are chosen by enthusiasts and workshops for their focus on protection, thermal stability, shift quality, friction control and application-specific performance.

What’s included: Select one size from the product options — either one 946ml / 1 US quart bottle of Red Line MT-90 75W90 GL-4 Gear Oil (50304), or one 3.78L / 1 US gallon container (50305).

Service notes: Remove the gearbox fill plug before draining where practical to confirm that it can be opened. Drain the existing lubricant, refit the drain plug with the appropriate seal where required and refill to the manufacturer's specified level. Do not overfill.