Gearbox "problems"

[DJexor]

I've got some gearbox problems after changing the oil this weekend. It's soooo hard to change the gears when the oil is cold. This morning I left home in second gear because I coudnt shift up from first to second. After 2 corners it wasn't a problem anymore, but shifting back was still no option. After 5-8 km's I was finally able to put it from 4th to 3th and 3th to 2nd. With the old oil it was also harder to shift when cold then warmed up but never like this!

So please help:

Old oil: ????
New oil: GL5 80W-90

Ive heared from Mux its not good to use GL5 for the synchro's, but could this also be the cause of the bad shifting?

Will change it again this weekend if it's nessesary, but what kind of oil would you use? (yea i know this question been on the forum alot...)