80-90 weight oil is typically used in hypoid rear differentials, not manual transmissions. Manual transmissions usually require a specific type of transmission fluid, often a multi-viscosity fluid like 75W-90 GL-4 or GL-5, depending on the transmission's specifications. Using the incorrect type or weight could severely harm your transmission.
Always consult your owner's manual for the correct fluid type and specifications for your vehicle. If you can't find the manual, a parts store employee or mechanic can help you determine the proper fluid for your 1996 Chevy Vortex's 5-speed transmission.