IMHO, it may be best to either one, do nothing other than add Lubegard or two replace only the filter in case the old one is restricting oil flow, catch and reuse the old fluid and still add Lubegard. The problem with new fluid is that if the clutches are on the razor edge of failing the new fluid will hasten that and putting the old fluid back won't reverse it.

Just curious is there any delay in the reverse gear engagement?

In the interim explore your best options for rebuilding the existing transmission (Sun Valley) or finding a good used one.

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