Choosing The Right Gear Oil: Part 1

Recommending the “right” lubricant for a manual transmission can be a touchy subject, just as it is when choosing the correct oil for your vehicle’s engine.

Aside from brand differentiation, which seems to be where most of the focus is when discussing oils, there are a few very basic ways in which gearbox oils differ from one another. These differences have to do with the way transmissions themselves vary: whether they have synchronisers or dog-engagement gears, whether they have an integrated final drive (and crucially what type of final drive), and what speeds and loads the transmission is expected to bear amongst others.

Before recommending specific oils, let’s have a look at a few of the most important factors that should determine an intelligent  gear oil selection.

 Viscosity is the number or numbers that indicate the pouring thickness of oils according to the Society of Automotive Engineers (SAE) standards. These numbers are almost always clearly indicated on the oil packaging, such as 75w90. While it is not necessary to know the meaning of the numbers specifically, it is important to understand what they indicate. 

 The higher the viscosity number, the “thicker” the oil. A thick oil forms a thick film around intermeshing gears, which acts as a barrier between them, drastically reducing friction and thus wear. This film is called an elastohydrodynamic (EHL) film. While a thick EHL film sounds great, there are, as always, trade-offs with the benefits of a thick oil. The main one is an increase in drag. A thick oil can cause a horsepower “drain”; one of the factors that contribute to driveline power loss. With an increase in drag, fuel efficiency also suffers. 

 Extreme Pressure (EP) Additives are the way that oil manufacturers offset the downside of using a thinner viscosity oil (i.e. a thinner EHL film). EP additives form a protective layer of deposits around metal surfaces in conditions of heat and, you guessed it, extreme pressure. This obviously protects the gears as they move across each other, reducing friction and heat, and thus wear. EP concoctions vary between oil manufacturers and are closely guarded as proprietary information. They are very evident in oils like Redline Shockproof A.K.A. “smurf’s blood” which has high ratios of Teflon-like EP additives, and are what make oils “slippery”.

Like anything in life, too much of a good thing may not be such a good thing. The very reason that EP additives are put in oil , namely to reduce friction, can cause a whole set of unwanted side-effects. EP additives are so slippery that they can prevent synchroniser blocking rings from doing their job. Synchros slow down the concurrent gear to enable a smoother shift, but EP additives can hinder this action as the entire synchro operation relies on friction. Even worse, some EP additives react with copper-based metal, just the material that most synchro rings are made of, causing corrosion and potential gearbox explosion, although these additives are being phased out. So how do we know which oil will work for our ‘box?

We’ll have a look at this question and more in Part 2, coming soon.


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