Have you tried "blipping the throttle" before trying to put it into 2nd?

The reason I ask, is the heavier gear oil doesn't allow the 2 different gear speeds to mesh as easily as ATF will.

80/90 is too heavy. ATF is basically 5wt oil. I use a blend of 30wt and 3wt. Shifts like a knife through butter. Chrysler actually recommended ATF in the FSM in the 70's.

What you can't see..........is the "cones" that are internal to the synchros and they have the all important job of creating a necessary interference to slow the gear speeds to match. If they are shiny, they are too smooth. Synchro's actually have a machined, finely grooved, surface to work with the mating cones and slow the gears. Thicker oil is counter productive in concept to what they are trying to do.