Does gear oil have to be synthetic?
Gear oil does not have to be synthetic. Mineral-based gear oils can be used if they meet the vehicle’s required specifications, and synthetic or synthetic-blend options are common because they often offer better performance under extreme temperatures and loads. Always follow the manufacturer’s guidance for your specific drivetrain.
Understanding when synthetic matters versus when a mineral oil will suffice hinges on the vehicle’s design, operating environment, and service intervals. Gear oil lubricates manual transmissions and differentials, and its performance depends on viscosity, standards, and compatibility with seals and clutches. The decision to use synthetic should be guided by the OEM specifications and real-world conditions like climate and usage patterns.
Understanding gear oil basics
Gear oil lubricates gears in manual transmissions and differentials. It comes in mineral, synthetic, and synthetic-blend forms, and it must meet specific industry standards and viscosity requirements set by the manufacturer.
Types and specs to know
Before choosing an oil, check the vehicle’s service manual for required viscosity (for example, 75W-90) and the recommended performance spec (such as API GL-4 or GL-5). The choice between mineral, synthetic, and synthetic blend is guided by these specs and by operating conditions like climate and load.
- Viscosity grade: Common gear oils include 75W-90, 80W-90, and 85W-140; the right grade depends on the vehicle and climate.
- Performance spec: API GL-4 or GL-5 for many manual transmissions and differentials; some gearboxes require GL-4 only, others GL-5; in some high-performance units, GL-5 with extra friction modifiers is used; Always match the spec to the manual.
- Oil type: Mineral-based gear oils are fine if spec allows; synthetic oils (PAO, esters) provide better oxidation resistance and temperature stability; synthetic blends lie in between.
- Clutch compatibility: For motorcycles and some manual transmissions with wet clutches, certain oils must meet JASO MA/MA2 or other compatibility standards.
Conclusion: The key is matching manufacturer specifications rather than defaulting to synthetic. If the manual doesn’t require synthetic, mineral-based gear oil can perform adequately for normal service.
When synthetic makes sense
Even when mineral oil would meet spec, synthetic gear oils can offer practical benefits in today’s vehicles and operating regimes.
Scenarios where synthetic is advantageous
Consider synthetic or synthetic-blend gear oil if you face extreme temperatures, high-load or high-heat operation, long service intervals, aging equipment, or a vehicle designed to run synthetic to maximize protection and efficiency.
- Severe temperature ranges: Cold starts in winter or hot summer driving reduce gear oil viscosity fluctuations with synthetic blends.
- High-load/high-temperature duty: Transmissions and differentials in heavy towing, performance cars, or off-road use can benefit from better oxidation resistance and film strength.
- Longer drain intervals: Synthetic oils tend to resist breakdown longer under heat, enabling longer intervals as per manual.
- OEM recommendations: Some modern vehicles explicitly call for synthetic gear oil or synthetic blends for optimal performance.
Conclusion: Synthetic gear oils are often advantageous but not always mandatory. Follow the manufacturer’s spec and weigh the cost against the benefits in your climate and usage.
Practical tips for choosing and changing gear oil
Choosing the right oil and performing a drain-and-fill correctly matters as much as the oil type itself. Here are practical steps to ensure you do it right.
Steps and considerations
- Always consult the owner's manual or manufacturer spec before buying; look for the exact viscosity and API/ACEA or OEM approvals required.
- Do not mix mineral and synthetic oils; if switching types, perform a full drain and rinse as recommended by the manual.
- Use the correct quantity; overfilling or underfilling can cause gear damage or leakage.
- Replace drain plugs and gaskets if advised; ensure seals are in good condition to prevent leaks.
- Follow the recommended drain interval based on usage; severe service intervals may differ from normal cruising.
- Dispose of old oil properly; recycling is important for environmental reasons.
Conclusion: Accurate specification and careful maintenance are essential to keep gears lubricated and protected over time.
Summary
Gear oil does not have to be synthetic; it must meet the vehicle’s prescribed viscosity and performance specs. Synthetic oils offer clear benefits in extreme conditions and for some modern transmissions, but you should always follow the manufacturer’s guidance. When in doubt, consult the manual, choose the appropriate oil type and viscosity, and perform maintenance correctly to protect manual transmissions and differentials.
