I recently changed the oil in my 700-R4 transmission on my 87 GTA after sitting on blocks for 6 months. Before putting the car on blocks the tranny shifted fine including going into overdrive. After replacing the transmission oil it will no longer shift into overdrive, all other gears shift fine. I just noticed I filled the tranny with Valvoline Dexron III Mercon oil, the manual calls for Dexron II. Could there be a difference in the oil weight causing Hydraulic pressure issues preventing it from automatically shifting into OD?
Or are the two oils compatible and this might just be a simple TV cable adjustment fix?

Thanks,

 

All versions of Dexron are compatible with earlier versions.