For dal, any oil will work because it is a fairly minor flavor component of the dish. The other flavors (I assume onion, garlic, and spices) will predominate. I have used vegetable oil, grape seed oil, and olive oil when making dal, all with no problems.
Mostly it depends of what kind of Dal you are making. But generally for dal you can use any Oil , Ghee or Butter. But more preferably Sunflower oil. You'll love the taste and aroma of the dal once its done.