A cryogenic expansion turbine system (10) includes a turbo-expander (12) configured to receive and expand a cryogenic gas feed stream (24). A rotary shaft (16) operatively connects the turbo-expander and a resistance device, such as a compressor (14) or brake. A bearing housing (18) has a bearing cooling fluid inlet port and a bearing cooling fluid outlet port. Electro-magnetic bearings (22) are positioned within the bearing housing and rotatably support the rotary shaft. A bearing cooling circuit (72) directs a stream of bearing cooling fluid (62) into the bearing housing via the bearing cooling fluid inlet port so that the electro-magnetic bearings are cooled and resulting warmed bearing cooling fluid (68) exits the bearing housing via the cooling fluid outlet port.

A lubricant fluid comprising a base oil, which utilizes carboxylic esters of carboxyl di-end-capped-polyethylene glycols, or a mixture thereof, to have ultra-low viscosity, high flash point, high viscosity index (VI), excellent low temperature properties (viscosity and pour/freeze points), and low elastohydrodynamic (EHD) shear strength, and to enable the production of high efficiency fluids with enhanced temperature properties for machines or machine elements that operate in the elastohydrodynamic regime of lubrication.