Hi I'm new to the forum. I bought my '61 Bird while working at a Chevy dealer it was traded in sight unseen. Lucky for me it scared the sales manger into wholesaling it. I offered to buy it and got it for less than the trade in was credited for. Apparently 1 of the other mechanics nearly got ran over when the car jumped into reverse. Hmm.. seems he had the steering wheel centered idling in park. I guess I won't be replacing the trans after all.
Spring of 1998 I flushed my cruise-o-matic with Dextron III. While my Odometer does not work I commuted to work 30 miles a day for 2 years, and now 5 miles a day. Approximately 20,000 miles total.
The car sat for 10 years and the transmission had been rebuilt before I owned it, but I don't know when. I did replace the filter and adjusted the bands witch were way out of spec.
I'm not recommending Dextron III, I'm just sharing my experiment. Please use your own discretion.