They recommend you change the oil every xxxx miles, probably a good practice.  I've changed mine once and that was because one of the seals started leaking.   YMMV......

Best regards,

Its pretty easy, some disassembly needed, but way easier than modern forks.

Id be tempted to do it if you have signs of leaking, but if you have it mostly sitting its probably fine unless you want to adjust the ride.

Lighter fluid will make it a little softer but run through the travel a little faster, heavier fluid will make it a bit stiffer.  The amount of the fluid can affect your bottoming out a bit too.  If you suspect any issues go for it.  

Otherwise wait till you want to do some maintenance and replace the sliders, seals and all that.

additional oil will make it stiffer, but there's a danger of blowing the seals out.

thicker oil will increase the damping

here's gary's thread on suspension...
http://suzukisavage.com/cgi-bin/YaBB.pl?num=1498156004