<<long story short: I put both a quart of ATF type F and a quart of ATF
Dexron III in together, one after the other, this evening. Both were
Chevron fwiw. How fast will that cause a problem? Is just sitting in the
gearbox going to do damage?>>

Alan,

Hurry up and get the rest of the Dextron out of the trans !!

When's the last time you serviced it?

Changed the more than likely plugged up sump screen?

If the filter is plugged the pump could be starving for oil, and
howling.

Use more type F.

It's an old, old, transmission trick shifting solution.

In leu of using B&M's Trick Shift type F is the next best thing for
crisp. smooth, and non slipping clutches in any automatic trans.

The type F has more grabber's in it.

The Dextron is the enemy.