Hi Peter
I use White spray on Lithium grease for the leadscrews and gears. Seems to work OK. I have also tightened the gibs on the z axis, and still lock them before milling. Your mod looks like a neat way of automating the z axis.
On a slightly different topic, I have been using small endmills for slot cutting at around 2000 rpm. I have found that after about half an hour the motor gets quite hot. I have remedied this by removing the cover and fitting a mains powered fan with ducting to accelerate the airflow through the motor. It may be that the taper roller bearings on the spindle are a bit tight, but I would rather have them this way.
Best Regards
picclock