Although I do not fully agree to not run low viscosity fluids, I am a fan on castrol imv in older vehicles.Do not use any type of LV fluid in that old spec transmission!
Use the Castrol Transmax Import Multi-Vehicle and add a bottle of LubeGard Red at the last fill.
One drain/fill is not sufficient, I personally do three, separated by 200 mile drives, to be sure that I have fresh fluid.