Frequently Asked Gear Oil Questions

1. What does API GL mean?
API stands for Americam Petroleum Industry and GL stands for Gear Lubricant, see below for their definitions:

  • API GL-1 Straight mineral oil.
  • API GL-2 Mild EP for worm gear oils.
  • API GL-3 Mild EP for spur and spiral bevel gears in axles and transmissions.
  • API GL-4 Medium EP, moderate severity hypoid gears, manual gearboxes.
  • API GL-5 High EP, all hypoid axles, some manual gearboxes.
  • API GL-6 Extra high EP, now obsolete.

2. Is it important to select the right API GL rating?
Selecting the correct gear oil performance will provide the best protction to the components of the transmission.

3. What do the SAE grades mean?
SAE stands for the Society of Automotive Engineers. The SAE classification system is a way of defining how thin or how thick an oil is. This is known as an oil's viscosity. The classifications are listed here in order of increasing thickness: ASE 75W, SAE 80W, SAE 85W, SAE 90, SAE 140, SAE 250.

4. What does EP mean?
EP means extreme pressure and refers to the additive used in gear oils. This additive is designed to stop metal-to-metal contact taking place between transmission components. The EP additives are usually based on sulphure and phosphorous. These elements bond to the metal surfaces where there are points of extreme pressure and temperature, forming a sacrificial chemical layer. The sulphur gives gear oils their characteristic smell.

5. Will synthetic gear oils and mineral oils mix together?
but beware, there are two kinds of synthetic gear oils available: polyalphaolefin (PAO) based and polyalkylene glycol (PAG) based. PAOs are basically a man made version of mineral oils (although with greatly improved properties) and can therefore be mixed with mineral oils. In fact, semi-synthetic products have mineral and synthetic (PAO) base fluids in them, so obviously, they must be able to mix. PAGs, on the other hand will not mix with PAOs or mineral oil. Utmost care must be taken when using this kind of product.

6. What is a hypoid axle?
Hypoid is an abbreviation of hypocycloidal and relates to the geometry of the crown wheel and pinion arrangement usually on rear wheel drive cars and vans. The pinion is usually highly off-set to reduce propshaft intrusion into the passenger compartment.

7. Do I need a special oil for limited slip differentials?
When the power distribution between two drive shafts is no longer equal (usually due to the surface condition that the drive wheels are turning on, i.e. ice, mud) limited slip differentials are able to effectively lock the two half shafts, ensuring equal power distribution. When this limited slip differentials 'kick in', there are high shock loadings on the clutch mechanism that requires protection from wear and slippage. The use of incorrect oils can lead to clutch degradation and vibration.

8. Why should I choose non-EP straight oils for my classic car?
Depending on the age, make and model, non-EP gear oils may be required for use in gearboxes and final drives. Certain designs contained a lot of phosphor bronze (copper containing) components that are sensitive to the sulphur-based extreme pressure (EP) additive. The sulphur attacks the copper and destroys the integrity of the meshing gear surfaces.

9. Is it alright to use ATF in a manual gearbox?
Certain designs do specifiy the use of an ATF fluid (automatic transmission fluid), but they should only be used where it is clearly stated by the manufacturer.

10. Is there one oil that will meet all of my requirements?
This will depend on makes and models, but very often the answer is no. Gearboxes, final drive, transfer boxes, etc. all have their own specific lubrication requirements. The specification of the oil required will be outlined by the design engineers, who will determine which type of oil will provide the maximum protection to the transmission components. It may certainly be possible to rationalise and reduce the number of lubricants used, but the magical single product may not be achievable.

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