The stock spring is the "medium" for people weighing 160-180 pounds. You obviously need to skip right past the "firm" and go for the the "extra-firm" spring. The bike shop you got the bike through should have set you up with the correct spring. They cost about $35 her in the US. Any Rockshox dealer should be able to get you one. Changing to a higher viscosity oil will not solve the problem of a badly undersprung fork.
Nice bike BTW. Looks like mine.
Nice bike BTW. Looks like mine.