...or at least look at... that your Z axis lub metering valves are clogged or damaged somehow. I recall a comment from a Fadal tech about it years back. He claimed that could happen if the incorrect lub oil was used at some time in the machines life or just plain dirty oil got in the system.
I may be wrong but, I think the valves for each axis are adjusted to allow for the different pressures required to lub at the different heights. The Z axis line has the longest path to push the oil, so it I imagine they would show the worst clogging first
This is all hearsay or guessing on my part, it has not happened to me.