Mustard oil. Siddharth Dasgupta provides fascinating information. The pungency of mustard oil is from a compound called allyl-iso-thio-cyanate. This is attractive to Bengalis but others may find it too pronounced. Unfortunately mustard oil also contains erucic acid, which is a low grade neurotoxin. It turns out that long term heavy use of mustard oil is injurious. All mustard oils imported to the US from India or Bangladesh) have to be labelled "For external use only". For many years Australia tried to grow a low erucic acid variant, but this was not successful.
Rape seed, a relative of mustard seeds from which the mustard oil was extracted, has similar beneficial properties. Mustard oil has equivalent benefits to olive oil in terms of health and, like olive oil, has a very low smoking point. Rape seed oil has similar properties but does not contain either the pungent chemical or the injurious one. Rape is grown successfully in Canada, but under the name Canola as rape is not an acceptable name. It is called rape in the UK.
Because of the injurious chemical, consumption of mustard oil has decreased. It will still be used in special dishes such as "tel koi" and "begun pora". At the same time the use of ghee has decreased for health reasons.