Which type of oil is sometimes used instead of 80w-90 gear oil in manual transmissions?

In manual transmissions, ATF (Automatic Transmission Fluid) is often used as an alternative to traditional gear oil like 80w-90. ATF provides several benefits due to its formulation, which includes additives for better lubricity and improved flow characteristics at lower temperatures. This can be particularly advantageous in manual transmissions where smooth shifting and protection against wear are essential.

Motor oil, while it is designed for engine lubrication, generally lacks the specific friction properties and additives needed for the requirements of a transmission system. It is not typically recommended for use in manual transmissions because it does not provide the same level of performance that ATF or dedicated gear oils offer.

Brake fluid has a completely different purpose, serving to transmit force in the brake system and is not suitable for use in manual transmissions. It also has different chemical properties that can be damaging to the internal components of transmissions.

In summary, while some might consider motor oil for use in manual transmissions, ATF is preferred due to its superior performance characteristics tailored for the demands of transmission applications.