I had regular dyno transmission oil in my new five speed .457 tranny. I could
NOT shift into second from first until the tranny had warmed up during cold
weather, then it would shift fine.
I would start the van let it idle for about two minutes (until the cold idle
stabilizes, I am replacing the idle module soon) then drive away.  I cannot
get into second until 5 minutes down the road.

I have put in Redline MTL gear oil in and now I can shift into second gear
with NO problems when the tranny is cold.

Robert