Animal Doctor

This is unlocked by completing the seventh mission of the game, which begins directly after your first milk run mission.

You'll meet a lady name Simone who wants you to clean some ducks at the lake, so begin by picking up the three cleaning items around the outside of her house.

Follow the icon to the lake, where you'll see ducks marked with a blue aura around them. Keep interacting with them until all are oil free and you unlock the achievement.