Brent 89-GT

I am running just regualr old GL-5 Valvoline dino oil in my GT's gearbox. It is a little slow when cold and you do need to be fairly deliberate at all times but I think it shifts very nicely. Don't tell anybody but.... the time I took it to the drag strip I was making foot to the floor upshifts with no problems, they can be shifted very quickly.

Mine had the shifter ball cup replaced by the PO and it has a GTS clutch. I doubt you have a trans problem, the GTS box is pretty tough. I would replace the ball cup though, that can give you just enough slop to make it seem like there is a big problem. I also think that the dogleg 1-2 shift makes being smooth tough.