Never substitute SAE Crankcase oil for SAE Gear oil. Gear oil has EP additives that protect the gears from making metal on metal contact and wearing out.
Always choose an oil that has the appropriate GL ratings. Check your owners manual for their recommended GL rating. If you are exceeding the rated torque capacity of your drivetrain you should consider increasing the viscosity of the oil and GL rating to a higher number.
Pay attention to all your drivetrain temperatures. It wise to purchase a infrared thermometer and record temps after each track session.
The higher drivetrain temperatures that are seen in road racing applications generally require higher viscosity oils.
Vehicles that are being road raced require race car maintenance and all drivetrain oils should be inspected after each race and preferably changed.
Listed below is an oil viscosity index chart that shows the viscosities in relation temperature.