The final drive has no filter so IMO it's better to change the oil more frequently than less. A regular TO-4 SAE50 is fine as regards lubricating properties, so I would go for a straight mineral oil and change it more often to get the particles out of the system. I can't believe it's hard to get a straight mineral SAE50 oil.
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I agree with Nige. I would rather put in cheaper oil and change it more often for the reasons stated above.