Internatonal 300 utility tractor transmission additive

[Cass Toon]

I just purchased this tractor and an Operator's Manual. For the transmission it says to use one quart IH Torque Amplifier Transmission Lubricant Additive to each of four gallons of SAE 10 or SAE 10W engine oil. Where do I get IH Torque Amplifier Transmission Lubricant Additive or is there an acceptable substitute?