I used 2.5w Motorex fluid when I did a fork service. Not much if any difference felt. One quick and easy improvement was going to SKF fork seals and wipers, not huge difference, but a little better (some of the new KTMs now have SKF seals as standard equipment, and is it listed as a model improvement)
If you are in there anyway to do seals though might as well try out removing some of the valve stack thickness. At the time I did mine I was not confident enough to try experimenting, but next time I'm in there will try to use lighter stack. Also it uses triangular shaped valve shims so I don't know how easy they are to find same style but thinner shims.
As for cheap, remove pre-load is free. SKF seal kits are a little pricier than the standard WP ones but worth it to me. They claim to last longer too