A lubricant is simply circulated through the expander with your working fluid.

Air Squared recommends approximately 5% (by mass) for lubricated operation. Most synthentic polyolester (POE) refrigeration lubricants should be acceptable. However, the following brand is recommended:

Emkarate RL 32H – Synthentic Polyolester Refrigeration Lubricant

Slowly inject the lubricant with inlet gas. If concerned about oil damaging components downstream, use an oil separator and sight gauge at the drainage plug in the bottom of the expander housing. If significant pooling occurs, the drainage plug can be used to drain. An oil pump can also be integrated to recirculate oil from the drainage plug to the expander inlet.