manual previa shifting issue

[Jonny]

My 91 manual Previa has the following issues....
When warm or in Summer when cold temps aren't that cold everything works fine.
In Winter, before everything is warmed up I cannot get it into first unless the vehicle is at a complete standstill and it won't go into 2nd without a bit of gear clashing. No problems going into 3rd, 4th or 5th when cold. When warm it's all good.
I had put this down to 'worn components' but since my Pickup has the exact same symptoms I'm wondering if it could be the oil I'm using.
The spec is 75W-90 and I'm using 80W-90 Castrol limited slip. Could this be my issue? If I change the oil to check the theory I'd like to go to something lighter to really make sure I can tell a difference. Would using something with a 70W rating be harmful??
jw