Any mineral oil.
Don't lose the little ball bearing that gives you the 'click' or detent every half turn. Spread an old sheet on your work table - better yet, a clean light colored towel, and take the preload apart on that surface. When the ball drops out, or is pushed out by the spring, it will not roll far.
When I did mine, I removed the bolts at the top of the hanger, then loosened the banjo bolt and removed the hose from the unit. Then I could take the preload assembly to my workbench for disassembly.