You should not try to guess. The best approach is to:
1. Identify the transmission type: This is crucial. Look for casting numbers on the transmission case itself. This will help determine the correct capacity. A photo of the transmission identification tag would be invaluable in this process. Online forums dedicated to classic Chevrolets can be very helpful in this identification process.
2. Drain and refill: The most reliable way to know how much fluid your specific transmission needs is to perform a drain and refill. Drain the existing fluid, and then measure how much came out. This gives you a good starting point. Add more fluid slowly, checking the level (most 4-speed manuals have a fill plug near the top of the transmission case that will allow the fluid to reach the proper level if topped off).
3. Consult a repair manual: A shop manual specific to your year and model Impala will provide the best information, including the correct transmission type and fluid capacity. These manuals often include detailed diagrams to aid in the process.
Important Note: Using the incorrect type or amount of transmission fluid can severely damage your transmission. It's best to be careful and thorough.
