Depending upon the material pair and load conditions, high acid number lubricants can reduce friction and increase wear.
See https://books.google.com/books?id=vkYCvwxyvsIC&pg=PA73&lpg=PA73&dq=effect+of+acid+number+on+lubricant+performance&source=bl&ots=StNYVL6y_r&sig=w3xOvVV4_sscY33YzPPQ5QyW1ag&hl=en&sa=X&ved=0ahUKEwjik6DGgtTTAhVDyoMKHfO1BFgQ6AEIUjAI#v=onepage&q=effect%20of%20acid%20number%20on%20lubricant%20performance&f=false