In the defense of the good man that is, Jonko Witter, I offer these quotes.
greenvoid says:
Then he produces a flask from his belt, and rubs some oily liquid into his hood and robe's backside from it.
This will keep off the burn of acid - for a while. Don't get cocky, though, it's not real armor. - and hands it over to Jonko.
and
Nikcollini says:
Despite his attitude, he uses the oil once Jonko finishes and hands it over. Then he passes it along to Zeno.