Instead of flush, change sequentialy.

This eliminates the question of "was a flush too much, too fast for an older transmission...now my tranny is slipping" as well as the possibility of creating a leak at the cooler lines.
(not questions to be belittled given what I've read in posts over the years)

Put it up on Rhino Ramps.
Pull the Tranny Plug, Drain and replace the plug.
Reach WAY down and pull the dipstick that Volvo designed incredibly difficult to access to keep Yahoo's from over filling thier transmissions.
Using a REALLY long funnel and fill with 3.25 quarts of Tranny fluid (Dexron-Mercon)
Repeat once a week for a month and then with your oil changes, say...twice a year.

Synthetic? ... to me you'd be tempting the leak gods... In the life of your tranny I'd say there's nothing to be gained with synthetics, except the real possibility of uncovering imperfections that only synthetics can get through.
I've put 245K on my 240 and the tranny is solid as can be with good old non-synthetics. Your not racing this thing in the stock division at Lime Rock are you? Do you live in Alaska or a God-forsaken section of Wisconsin where you MUST plug the car in in order to start it in the winter? No? OK. Then use a brand name Dexron-Mercon and be done with it. Use the cash you saved on the synthetic to do something fun.

Tom