I have had a Clausing 5914 lathe since 2007, and always had problems with the variable-speed drive, and a major problem was that I never could get all the air out of the hydraulic system that is used to control speed.
Following Clausing's recommendation of the day, I have been using Mobil DTE24 Hydraulic Oil in the control system and headstock.
I tried many things to bleed the air out of the hydraulics, and sometimes I's succeed, but it would not last. I could hear air being aspirated back into the system when I pushed the control wheel to the low-speed limit, so I was just chasing my tail -- all motion, no progress.
I was going to solve the aspiration problem by adding an external oil reservoir (like the old cylindrical steel upper hydraulic assemblies) so that the path to the free surface of the oil would be too long for aspiration to be possible, so I inquired on the Clausing list if anybody has a rectangular aluminum upper assembly for sale.
This elicited many questions, and a clue. It turned out that oil product names and numbers changed between when the lathe was made (circa 1975) and 2007, and the old oils (Shell Tellus 27 and 33) were not near-equivalents to the similarly-numbered Tellus 22 and 32. What had happened is that the oil industry eliminated the profusion of different ways to measure oil viscosity, and settled upon the ISO Viscosity definitions and grades. The new Tellus product numbers are based on the ISO Viscosity Grades of the oils, and not on whatever had been used before the transition.
Clausing Tech Support fell into this trap circa 2007. The effect of the error was that the recommended substitutions were an entire viscosity grade too thin.
New (2015) substitutions:
Tellus 27 --> ISO VG 46 (Mobil DTE 25, not DTE24).
Tellus 33 --> ISO VG 68 (Mobil DTE 26, not DTE25).
Changing the oil in the headstock and hydraulic system to DTE25 had a dramatic effect. Now, purging air is easy, requiring one maybe two cycles (versus ten or fifteen, which nonetheless often failed) and the VS system works correctly. And in a welcome surprise, the lathe runs far quieter now. (I also changed the oil in the apron to DTE26, but there was no obvious difference, although it was in any event due for an oil change.)
I also now know why the old purge approach sometimes worked and sometimes did not -- it depended on the season of year: Oil viscosity is very sensitive to temperature, so in the winter the oil is thick and the purge cycle worked, but in the summer the oil is thin and it did not work.