Upgrading the front suspension in the 01. Already ordered a set of .85kg/mm sonic springs and so now my question is about oil. First off for my own knowledge, What does the weight of oil do to the performance of the front fork? for example 5w oil vs 20w oil.

secondly in your opinion for a aggressive street / cruiser bike what weight should i run. Current body weight at 185-190 with gear on.

Good question.  I know that the GS500 manual calls for 10W fork oil.  I also know that the height of the amount of oil in the fork can be varied for a softer or stiffer ride. 

more oil = stiffer

Not sure what effect the weight of the oil has however...

im not sure either, but off the top of my head i'd say it would affect compression rate. IE: faster or slower compression

That seems to make sense.  Higher weight = slower compression rate.  I noticed on one site that racing fork oil tends to be lower weight.  This seems logical since you would want your suspension to react quickly in a racing situation.  This is all conjecture until someone who knows what they are talking about replies .

I'm using .85 sonics in my 03 with 15w oil and it's magic you don't want to go too stiff.

Heavier oil will slow compression but it will also slow rebound (which can be a more noticeable effect).  Quick suspension 101 for anybody that doesn't know:  The 3 main terms you hear with suspension is compression, rebound and pre-load.  Compression refers to how easily the fork slides up.  Proper compression keeps the fork from bottoming out over large bumps.  Rebound refers to how quickly the fork extends after being compressed.  Preload has to do with the amount of force it takes to start the fork to compress.  If you ever hear people talk about "setting sag" they are talking about adjusting the pre-load.  Oil level primarily effects bottoming resistance and fork stiffness in the latter part of travel.

The weight of the fork oil will affect the damping (how quickly the suspension reacts).  A heavier oil will make the suspension compress slower but it will also make it slow to expand (rebound).  Slow rebound can be a problem on a bumpy road because the suspension may not be able to fully extend from one bump before it hits another.  10 weight oil is standard on a GS.  That should be your starting point with your new springs.  If the front end feels bouncy, switch to a heavier oil.  If it feels too stiff/slow, switch to a lighter oil.  Dexron III automatic transmission fluid is about a 10 weight oil.  Ford Type F trans fluid is about a 7 weight oil.  They are both cheap alternatives if you don't have ready access to real fork oil.

-Jessie

The preference in West Seattle seems to be 0.85's and 15w. Everyone is happy with that combination. Never hear a complaint about the front set up that way.

Good choice!