downshift in cooler weather -help?

A727, reverse pattern maual valve body. no warmup tank on aluminum rad. and oil cooler in front of rad.
took the car for a spin on the weekend 45-50*F outside. warmed the engine up (Airgaps are not cooler/cold weather friendly) for at least 10 minutes.
Took it real easy and shifted up to 3rd no problem. Down shift from 3rd to second and the tranny kinda hung up - shifted back up to 3rd no problem.

Is this a case of the oil (standard weight Dextron) being just too cold to flow
into the right places - or am I headed for the tranny shop.
I plan on changing the oil before next excursion and was wondering if I should go Synthetic?