I definitely recommend baby oil, although you can do it purely with patience. When I had my mare, my family and I spent a bit of time with the land owners' horses, one of which was an incredible clydesdale x thoroughbred mare with a mane to die for, except it was full of burrs and knots. My mother and I set to work with combs and in about a half hour we had a good chunk of her mane untangled and beautiful.
As for using baby oil, I used it on the tail of a standardbred who had spent the last few years just standing in a paddock. Rub it in as well as you can to coat the hairs, and you can start detangling it with your fingers. Once you get the worst of it out, get a comb into it and start separating the hairs in small chunks.