Manual transmissions usually take 90-weight gear oil, engine oil, or automatic transmission fluid. The manufacturer determines the correct fluid to use in a manual transmission and usually prints this information in the owner’s manual and on the gear box.
The different types of fluid have different thicknesses and viscosity that make them suitable for certain types of transmissions and manufacturers design transmissions to be used with only certain types of fluid. Transmissions that require high heat tolerance and high viscosity tend to require 90-weight gear fluid, which is thicker and more viscous than automatic transmission fluid, which is more watery and able to penetrate small parts more easily but not heat tolerant. Using an automatic transmission fluid in a manual transmission designed for 90-weight fluid causes the fluid to degrade rapidly and the transmission to burn up. Using a 90-weight fluid in a manual transmission designed for automatic transmission fluid prevents the parts of the transmission from moving easily and causes the transmission to fail.