Pass this around.
Hooked a vfd to my manual lathe so we could run on single phase. worked great then one day we wanted to go slower for some large tapping. We reduced the speed of the motor using the vfd.
Several weeks later someone noticed there was no oil showing in the head-stock window. After checking the oil level and the pump which were all ok. We discovered the problem was the oil system is a non pressurized fountain type system were oil is pumped up to the various levels in the gear and bearing system and just overflows at the top. Which is your spindle bearings and the window.
So if you slow the main motor down you slow down the oil pump. At lower speeds it does not supply enough oil to get to the top level.

End result if we didnt catch it,would eventually be a worn out bearing set.
So pass this around to your mori lathe guys and save them some heartache.