In the service manuals for these motors there is a valve adjustment at 60k miles/ 100k km. I believe it's done by adjusting shims. Has anyone done this themselves or is better to have the experts do it. And is it essential to do? Anyone done it and noticed better performance after?

Valve lash adjustment on the 1998+ 3.2/3.5 sounds like a royal PITA. You can flip over & reuse the shims. General consensus is leave'em alone unless you get a steady, constant tick. Check this post: http://forums.delphiforums.com/itog/messages?msg=2653.9
Jerry gives some real good info.

G/luck
Joel

This Is Not A Do It Yourself Job Unless You Have Some Serious Experience