Doing what you propose certainly won’t hurt anything, but it seems excessive to do it every oil change, even if it’s only 1/5 of the total system volume. Generally, keeping the power steering system topped up is entirely adequate. The Toyota dealer should be able to supply the Dexron-compatible fluid that they use in their service department. If you’d rather buy it yourself, any premium brand that is Dexron compatible will be fine.
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