What to consider when choosing transmission oil?

The most common mistake when selecting transmission oil is reducing the oil to a single number. However, three separate criteria must be met simultaneously: viscosity grade, additive package, and manufacturer approval.

Viscosity grade

Viscosity defines the temperature-dependent fluidity of the oil. An oil that is too fluid cannot form an adequate film thickness on the gear surface; an oil that is too thick increases friction loss during cold starts and reduces synchronizer performance. Deviating from the grade specified by the vehicle manufacturer causes damage in both directions.

Additive package

In transmission oils, the additive package is determined by the gear type. While drivelines with hypoid gears require extreme pressure additives, the same additive in synchronized transmissions can reduce the friction coefficient and impair shift quality. Therefore, the assumption that 'higher additive oil is better' is incorrect.

Manufacturer approval

The approval list indicates that the oil has actually been tested in that specific transmission. Of two oils bearing the exact same grade on the label, only one might be approved; this difference is also decisive for warranty coverage.

Mixing

Mixing oils with different additive packages can lead to the additives neutralizing each other. If topping up is required, the exact same product must be used; if the product is being changed, the system must be completely drained.