PEX has only LIMITED chemical resistance vs. diesel fuel.

PEX has poor chemical resistance to Oleic Acid.
My view is that since 'reclaim' oils contaminated with Oleic Acid are becoming more and more common as unscrupulous fuel dealers / suppliers are diluting diesel fuels with 'reclaimed' cooking oils. When specifying materials used in diesel fuels, one should also check the materials compatibility vs. oleic acid - a byproduct when lipids become 'rancid'. Oleic is a very 'nasty' chemical vs. most polymers, except viton or fluoropolymers.