It's the best if one can develop the taste... it has the lowest saturated fat in edible oils. Mustard oil does have erucic acid, which is considered noxious and not fit for consumption in the west, but the style of cooking in India too involves heating it up to almost smoking that reduces its noxiousness and smell. Also , Mustard seeds have bronchial system healing properties. It seems to be good for preventing wheezing. This is from a study in the US.
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Mustard Oil (Kachhi Ghani) is widely used in Rajasthan and Eastern States, esp., WB as the cooking media. It is an ideal media for Fish preparations. However, in Kachhi Ghani Oil, the FFA (Free fatty Acid) level is too high, beyond the stipulated limit of 0.12%, hence it is not recommended for old people and heart patients. However, refined Mustard Oil/Rapeseed Oil has litle FFA and can be used by all and sundry.