Traditionally Bengali cuisine uses mustard oil. If you are not used it it, pl feel free to use oil of your choice.
The taste & texture of this dish depends a lot on how well the dal is boiled. The dal should be perfectly cooked, but not mushy.
The consistency of the dal depends on the main dish. If you want to eat it with rice, it can be a little watery. If its for Luchi, it can be a little thicker.