Which Oil Should Be Used in an Austin Healey Overdrive Gearbox?

austin healey overdrive gearbox oil

The type of oil that you should use in your Healey overdrive gearbox may be something you’ve thought of for a while.

The reason for this is often because the available information on the subject can be rather confusing. Below are some basic issues one man uncovered:

  • Overdrive gearboxes are operated hydraulically, thus the it’s the pressure that makes the gearbox operate; your oil will need to act as hydraulic fluid.
  • The overdrive gearbox has a wet clutch – it spends its life surrounded in oil
  • Syncromesh operates on the gears using friction
  • Temperature can be an in – mineral based engine oil will become thinner in lower temperatures than gear oil
  • Gearboxes pre BJ8 will have brass rather than steel syncros – any older EP mineral gear may have additives which attack  the brass
  • The oil has to be capable of lubricating the gearbox
  • Gearboxes won’t have a filter like a car engine would

Original literature has said the Healey gearbox and overdrive should be using a mineral engine oil. However, the exact same overdrive on a big Healey will also be fitted to a Triumph – which also states to only use gear oil.

If you would like to use a mineral based oil, many experts agree that brass syncro gearboxes should be using engine oil. This is only before some mineral gear oils have additives which may corrode brass syncros.

When using engine oil, avoid using a ‘friction modified’ engine oil. As the syncro cone relies on friction to operation, if the oil is too slippery (‘friction modified’) the syncro is likely not to work. This is the main factor of why people choose to use a mineral engine oil which doesn’t contain friction modifiers.

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