Why do farmers cover bodies in burnt motor oil before harvesting chestnuts?

When farmers enter stagnant ponds to cultivate water chestnuts/singhara, they are exposed to poisonous crabs and snakes. To protect themselves, farmers coat their bodies with burnt motor oil, which they usually get for free from nearby areas. That is their only protection against rashes, itching, and skin rot. Without oil, their fingers begin to decay from constant exposure to water.