fluid recommendations?

I have a 99.5 jetta, manual transmission, and I just recently added 75w90 synthetic valvoline fluid. It made my car shift smoother in the warm fall weather, but started to shift really rough as soon as it snowed and got cold outside. I almost have to force the stick into gears. It does fine when it runs a little bit and gets warm. BUt im afraid im going to ruin something one of these days when im in a hurry and cant let the car idle. What does everyone else use? I was thinking to try redline 75w90 or else royal purple 75w90?