The manufacturer recommends SAE 80 or SAE 80W-90 GL-5 Gear Lubricant as the proper lubricant in all three-speed and four-speed manual transmissions.
Check the lubricant level when the transmission is warm. Remove the filler plug from the transmission case from underneath the car. If necessary, add sufficient lubricant to bring its level to the bottom of the filler plug hole. Replace the filler plug.
DRAIN & REFILL
Remove the transmission filler plug. Place a container under the transmission below the transmission drain plug. Remove the drain plug and allow the lubricant to drain into the container. After the lubricant has been drained, install the drain plug. Add sufficient lubricant to bring its level to the lower edge of the filler plug hole and install the filler plug.