5, 10 or 15wt in the forks?

Q1:
Is there really a big diffrence between the viscosity on the fork oils?

Q2:
I mostly ride on MX tracks when it´s between 50 and 70 degrees F.
The bike is an -90 Y250 so i guess the forks are a bit old and soft...
The manual says 10wt, but maybe 15wt is better considering worn bushings and that sort of things?

Q3:
It has the original springs and My weight is 190lb, should I use a little more oil also?

thanks in advance!