Fork Oil

The std oil weight is 5wt

Different weight oils will certainly effect how your suspension behaves. Heavier oil weights will stiffen the damping (some say most noticeably the rebound damping) and increase the chance of cavitation during rapid suspension movement.

Running 15w in a bike valved for 5w would be a disaster!

ATF is supposed to be around 7w and has many characteristics that make it good as fork fluid (and it's usually cheaper).

I believe the Showa twin chamber fork uses 5wt in the inner chamber and 10wt in the outer(i never understood why-can someone enlighten me?)most modern forks use 5wt with some heavier/faster riders(or not so heavy) using something like mobile 1 atf which rates at roughly 7wt like stated before.The viscosity of the oil will have the biggest difference on low speed action of rebound and compression-like stated earlier probably more in terms of rebound.I dont see any reason for not using a 2.5wt oil in the forks if they are valved too stiff for the application.The rebound can be tuned using the clickers as thats what they are there for.I see fork oil weight as a effective tuning aid for everyone who doesnt want/trust or afford to have the forks revalved.

The oil weight are not a very good guide to the viscosity index of a oil so yes different 5wts can act differently-so i would say get a brand you like and stick to it-redline oils i think have some info on how other oils compare in terms of VI to the main competiton

I have not heard of the two weights in TwinChambers...hummm...where did this come from? Curious.... :think

As far as changing weights goes, what I have noticed is when going to a heavier oil (5-ATF for instance) that, as Marcus indicated the greatest effect was in the LS range. The clickers do have a greater "range" with the heavier oil though, meaning that each click is a slightly greater change than with the lighter oil, and that because of this, a couple of clicks of adjuster makes up for the slightly greater LS created by the heavier oil. In the higher speed ranges, the effect is minimal at best.