Can Someone Explain the Difference Between Hypoid and Non-Hypoid Oil?
By BRETT WINBERG

We’ll kick off this first issue of the New Year with an often asked question about gear oils. But before we get started, we would ask that your thoughts and prayers go out to the victims and families of the one of the largest natural disasters in history. Please help in anyway you can and learn more by visiting RedCross.org.

This weekend I had the privilege to change differential gears in a 4x4 pick up truck. As the weekend was ending, I asked my son to pick up some gear oil (80w90) for the project from the local parts store and to make sure it was for “Posi-Track” (Limited Slip) style differential gears.

While at the parts store, he had many questions for the counterman, but they were not much help. So I returned to the parts store to get the oil and help explain to the manager what the difference was between hypoid and non-hypoid type oil.

With assistance from industry insider, Andy Dingly, this should help you and your team to understand the difference of gear oils in today’s market place and if Hypoid is actually what you are looking for.

"Hypoid" is not really a question of oil, so much as a question of gear cutting. Old (1920's) rear axles used straight bevel gears to form the crown wheel and pinion. These had two disadvantages, the pinion shaft meets the crown wheel on its central axis, and the straight cut gears are noisy.

By using a more complex "hypoid" gear tooth shape (if you look at a pinion, the teeth appear twisted) these problems can be addressed. The more gradual engagement of the teeth along their length reduces noise. By careful design of the geometry the pinion can be made to mesh below the axis of the crown wheel. As the center height of the crown wheel is fixed by the wheel height, this allows the prop shaft to be lowered relative to the car body, giving a cleaner floor pan and lower center of gravity for better cornering. Hypoid bevels are now universal in this application.

Because of the sliding contact that hypoid gears make, their hydrodynamic contact pressure is higher. To be suitable for use with hypoid gears, a lubricant must be capable of resisting high pressures. Oils with "EP" ratings (Extreme Pressure) such as EP90 are required. Some brands describe themselves as "hypoid" instead, a term which is synonymous with EP. GL-5 is a formal API standard for this type of oil (comparable to MIL-L-2105B/C/D)

So, back to my installation... my owners manual is telling me to use Non-Hypoid gear oil 80W or 80W/90 on the manual transmission and GL-5 hypoid gear oil 90W in the rear axle. A manual transmission won't usually contain hypoid gears, so it doesn't need EP oil.

Rare exceptions are those transaxles where the crown wheel and gearbox share the same lubricant. Although EP oil is more complex to manufacture, it has no disadvantages when used in instances where the EP attribute isn't strictly required. Manual steering boxes and other slow-moving oil-containing components are often filled with 90-weight oil. It's usually okay to purchase EP90 because that's what the axle requires, then use the same oil for all other components.

There is little practical difference between 80 & 90 weights. There's an increasing trend amongst manufacturers to reduce the number of different lubricant types required. But not all gearboxes are the same, so make sure to read your owners manual. For example, a 5-speed Range Rover runs on ATF, but 20W/50 engine oil or EP90 axle oil is equally permissible, if the climate is of the warmer nature.

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