I wouldn't recommend using higher viscosity oil in a modern bike shock - rather use the correct 2.5W oil and re-valve the shock to give it more damping. I am using a 8.3KG/mm spring (100kg rider+ luggage for Angola trip) with re-valved shock. Works very well.
I didn't like those flimsy front forks too much and transplanted a set of WP48mm forks and triple clamps. Also had to re-valve, shorten and re-spring the forks. Feels like a real motorbike now. 
Hey Superfoxi, Ja, I agree this is definitely not the best way to do things but it gives pretty good results and very good bang for your bucks. I do want to upgrade the pretty basic stock forks when a suitable set comes along and revalve the shock (please send me the approximate cost for this too).
Out of interest, why wouldn't you use higher viscosity oil in a modern shock?