Getting to grips with your transmission – With Opie Oils

Hi all

It’s time for another technical post from your friends at Opies. So if you’re experiencing gearbox issues or simply want to get those gear changes nice and smooth, then you’ve come to the right place – pencils at the ready!

The lubricants used in a gearbox play a critical role in the operation, life cycle and performance of any transmission. Manufacturers are now prescribing “life time fluids” more and more often, however these can often give stiff shifting, excessive wear and noise if left unchanged for the duration. So, if you’re after some pointers on how to get the best out of your box, there’s some basics that you’ll have to get to grips with first. Have a look below.


Know your ratings

First of all, lets have a look at the basics.

The American Petroleum Institute set the recommendations for oil manufacturers to follow. Incidentally, this is where we get the “API” rating from. These ratings generally control the viscosity parameters and additive content for all oils produced for the automotive market. The two most common categories are:

  • GL-4: Designates the type of service characteristic of spiral-bevel and hypoid gears in automotive axles operated under moderate speeds and loads. Commonly used in Syncro-mesh manual transmissions and transaxles.
  • GL-5: Designates the type of service characteristics of gears, particularly hypoids in automotive axles under high-speed and low-speed, high torque conditions. Commonly used in differentials and dog/straight cut gearboxes.

The Chemistry

Base stocks are important, however this is not something that all lubricant producers readily publish or would like you to know. There are three main classes:

  • Group III: A refined mineral oil. This can be listed as a semi-synthetic or even a fully synthetic by some lubricant producers. Conventional mineral oils contain greater amounts of impurities, such as sulfur. You may also find reactive and unstable hydrocarbons, as well as other undesirable contaminants that cannot be completely removed by conventional refining of crude oil.
  • Group IV: A true synthetic – blended in a laboratory to achieve the intended end product with little compromise. Known as polyalphaolefins (PAOs) they are found in the majority of high performance oils available on the market. A broader temperature range and stronger film strengths mean these products are well suited for performance applications. They will take a real beating before the begin to sheer or break down.
  • Group V: Specifically ester base stocks, originally designed for use in jet engines due to extreme temperature stability and film strength. Ester base stock oils are perfectly suited to automotive lubrication due to the natural multi-grade properties – this eliminates the use of thickeners which become unstable under high temperatures.


Base stocks often determine the quality of the finished product and inevitably the price, as with everything you get what you pay for and there are many benefits of group 5 synthetic oils:

  • Considerably more fluid in low temperatures and start up
  • Oil stability is greatly improved during high temperatures
  • Maintain much greater stability through the viscosity range
  • Reduced oxidation over time means synthetics can operate for extended drain intervals
  • Greater wear protection under higher loads and speeds
  • Reduced friction throughout the drivetrain resulting in a more efficient package



Automotive lubricants are blended with an additive package. The additives determine the characteristics of the lubricants operation and can be altered for different applications.

A common anti-wear additive used is Zinc dialkyldithiophosphate. We know this looks like a bit of a mouthful, so it’s commonly referred to ZDDP. This type of additive literally reacts with the metal surface when the reaction energy (temperature) is high enough. This reaction layer provides a “sacrifical” surface layer that offers increased levels of protection.

Extreme Pressure Additives (EP) are used as the loading and metallic contact increase, the strength of the additive and reaction process increases. This leads to the use of a sulfur-phosphorus based extreme pressure chemical. The EP additives form organo-metallic salts on the loaded surfaces that serve as sacrifical films to protect against aggressive surface damage. EP additives are used in oils to reduce the adhesive wear of gears and absorb the shock of the moving components making contact. These additives essentially create a barrier or film between between the moving parts, its chemistry means the oil clings on to moving parts.


All in all, getting to grips with the basics of transmission oil and what your requirements are, will ensure you have got the best possible choice for your particular set up. At Opies, we have a huge range of products available from the likes of Millers, Fuchs, Castrol and Red Line so we’re sure you’ll find something suitable. You can find them all HERE. Don’t forget to use your Opie Oils blog discount “OPIEBLOG” which will give you 10% off your order!

We hope you have found this informative and helpful, but should you need any advice on the correct gearbox oil, or if you’ve got something a bit special then please give our experts a call on 01209 202 944 or drop us an email to and we’ll be happy to help. We’re in the office Monday to Friday 8:30-5:30.

Until next time




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