I would use grease since you can't easily oil them.
Ed
I'm not totally familiar with the particular machine but grease will hold chips and other abrasives, whereas frequent oiling will wash it away.
monkers,
What mill are we talking about in this thread?
Ed
Its a Jet JMD-18 mill/drill. I will use way oil on the ways, Im gonna drill the saddle in a couple places and install oil zerks. I will also use oil on the lead screws. For the rack on the back of the quill and the rack to raise and lower the head I will use grease. Im gonna install an oil cup to oil the fine feed worm and shaft.