1999 Isuzu Rodeo LS burning oil?

I have a 1999 Isuzu Rodeo LS, 4WD burning oil. I can see that it is not leaking any fluids, but at the back of the muffler pipe in the rear of the vehicle has dried, black oil in it. So I know it's burning it. I've added Lucus Oil treatment several different times to the oil when I've changed the oil, and it has helped, but it is still burning oil. I need to find out the best and cheapest way to fix it. Thank you.