Conditions of exposure and the presence of other chemicals and trace impurities are important when evaluating the compatibility of a material of construction. So these need to be considered when looking at a specific situation like yours.  In general, the main issue with glycols is the formation of acids by glycol degradation at elevated temperatures. This is why corrosion inhibitors are added to glycols used as heat transfer fluids. Having said this, 316L stainless steel is considered compatible with propylene glycol under many conditions and is a recommended material of construction for glycol storage. Pruett's Chemical Resistance Guide for Metals and Alloys (Compass Publications) gives 316L stainless steel an "A" rating for propylene glycol at concentrations of 30% to 100% at temperatures up to 195° F. For more information, you might consider contacting a manufacturer of propylene glycol and propylene glycol heat transfer fluids (e.g., Dow Chemical).