i used thick lube (krytox 205), but ended up removing it again..
even a small amount slowed down the strokes too much, and it ruined the typing feel.
can imagine that thinner lube (oil?) could be good. because all you really want is lessen that gritty feel that the strokes have.
but definitely keep stabilizers lubed up! mine came with a fairly sloppy lube-job, re-doing it made the sound and feel so much better.
thick lube here, is king. lube generously, for more points
edit: some audio clips imply that a good lube job makes the keyboard way more quiet. so i guess there's another good reason to lube your hhkb. i'm certainly gonna try it again, when i get some thinner lube : )