Therefore, specifying the correct fluid is impossible without more information. To get the correct answer, you need to:
1. Verify the year and model of your truck: Double-check the year, as even the model is highly suspect.
2. Identify the transmission manufacturer: The transmission likely wasn't made by Chevrolet. A tag or casting mark on the transmission itself may identify the manufacturer.
3. Find a transmission manual: Once you know the manufacturer and model of the transmission, you can search online or consult a manual for the specific recommended fluid.
Without accurate identification, any fluid recommendation would be pure speculation and could seriously damage your transmission. If you're working on a restoration or have found a very unusual truck, consulting a specialist in vintage Chevrolet trucks or transmissions is highly recommended.