I'm in the process of designing a high pressure hydrogen compressor. In this machine, an oil will be in direct contact with the hydrogen. To minimize the amount of oil vapor that enters the hydrogen and must be separated, an oil with high molecular weight and low vapor pressure should be best. Ref: EngineeringTalk The oil used in my compressor doesn't need to be a particularly good lubricant, it is used to displace hydrogen and compress it. My thinking is to use a vacuum pump oil instead of a conventional compressor lubricant. The vacuum pump oils have a vapor pressure on the order of 0.0001 micron. They can achieve this by distilling down the oil and removing the lower molecular weight parts. Edit (this part added): Another concern regards how much hydrogen might disolve into the oil. Does anyone know if hydrogen disolves readily in mineral oil? I haven't looked into compressor oil enough yet to know how they compare. Any comments or suggestions?