I have heard of using WD40, but I myself have never tried it.
What I have always done (horse had a club for tail, well...almost) is to go get the cheap Suave conditioner and saturate the tail with the conditoner. Wet the mane and apply gobs and gobs of the conditioner, to the point where you can almost ring the conditioner out like water. Let it sit for a 5 mins or so and then start hand picking it out, start at the bottom and work your way up. This works so much better than baby oil (all it does is attract dirt), show sheen (dries the hair out), mtg (cheaper, don't know if it actually works better since I have never used it). You could probably use coconut oil as well (when it is in the liquid form), since this is one of the best ways of conditioning hair (horse or human). I haven't not tried coconut oil for knots becuase I just found out about using the product for conditioning.