What should be avoided when selecting fluids for manual transmissions?

When selecting fluids for manual transmissions, it is crucial to avoid using brake fluid because it is corrosive and can damage seals and internal components of the transmission. Brake fluid is designed specifically for hydraulic brake systems and operates under different conditions than the fluids required for a manual transmission. Manual transmissions typically require specific gear oils or lubricants that provide the correct viscosity, friction characteristics, and protection against wear. Using brake fluid can lead to malfunction and significant damage to the transmission system, which is why it should never be used in this application.

Other fluids like coolant, engine oil, and motor oil also do not meet the requirements for manual transmission fluids and could cause similar issues; however, brake fluid's corrosive nature makes it particularly damaging and, therefore, the most critical fluid to avoid.