Manny manual transmissions use gear oil viscosity of about SAE 80W or 90 grade. This is a gear oil viscosity and has nothing to do with motor oil viscosity. For instance, SAE 80W gear oil can have the same viscosity characteristics as SAE 40 or 50 motor oil.
However, not all-manual transmissions use gear oil. For years, Chrysler Corporation specified the use of automatic transmission fluid in their manual transmission vehicles. Some transaxles, both foreign and domestic, use either ATF or engine oil to lubricate the transmission. For this reason, it is always best to consult your owner's manual or your dealer if you are unsure about what sort of lubricant to use in your manual transmission.