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First, I say this to help you and not to be a d!ck, but it's damping, not dampening. That will help you get more responses.
Fluid dynamics is a very complex subject, so a lot is trial and error. The damping of a fork is a function of : shim stack, piston design and yes, oil weight. Where it gets tricky is the combination. For example, I use Ohlins forks and they require Ohlins oil... usually I laugh at that because I think it's just because they want the extra $$. In this case it's true, the pistons are perfectly designed for that oil viscosity... complete crap in most applications, but perfect for that one. Also, all weights are not created equal... some 5W is closer to 8, some 15 closer to 12. There are many combinations that will work, but you have to do some educated guessing and trial and error.
IMO, adding heavier oil with a stock piston will not give you the desired effect due to the HMAS port design. If you use a racetech piston, heavier oil will do the trick (mix 50%US1 50% US2). Ktech or Ohlins pistons work best on Motul factorline 5W.
Hope this helps.