Gear Oil Weight
Viscosity of gear oil is as important as engine oil, the correct weight will elongate the life of transmissions. When a gearbox or differential is designed an oil weight is selected to work optimally with the components.
Synchronizer rings are heavily dependant on oil weight. If the oil is too thick or too thin excess wear will be caused on the synchronizer rings and they will not engage smoothly.
Bearings need the right amount of oil flowing to and from them otherwise they suffer from overheating and premature wear. To ensure the correct flow oil channels are designed to match the oil viscosity.
When designing the tooth profile of the gears, consideration is given to the weight of the oil. An oil that is too thin for a tooth design will cause the gears to make metal on metal contact, causing premature wear. Too thick and excess drag is caused, reducing economy and sapping power.
Gear oil is either rated as either a single grade or a multi grade oil. A single grade oil will have a rating such as SAE 90, and a multi grade oil will have a rating such as 80W-90. In these two examples the '90' refers to the viscosity when warm. The 80W refers to the viscosity when cold, the 80W-90 will have a thinner oil when cold than the SAE 90. Most modern transmissions are designed for a multi grade oil. However some older ones are designed specifically for using a single grade oil.
Unless in exceptionally hot or cold conditions you should always use the manufacturers recommended viscosity.