Like Curryqueen says, tarka is nothing like the curry gravy oil
The tarka oil has a mild tomatoey taste
The curry gravy oil is so strong that it dominates the whole curry's flavour
If you get it on your hands, then they stink for days
It is the main source of the BIR "flavour"
In all the kitchens, I have been in, it is the only oil they use (apart from scooping a little more from the top of the curry gravy too)
They have it salvaged, in a 6 inch high ghee pot, near (or on), the stove.
The next time anyone has a curry, pour a sample of this oil off.
Get a couple of teaspoons worth
The next day check it out
You won't believe how powerfull it is
It's flavour is not made at the time of your curry
It is from frying, at the start of the curry gravy, and then many hours of boiling afterwards.
To make a BIR style curry you really only need three things
A really good (but watery) curry base (using black cardamoms)
A good spice blend & curry gravy oil
As soon as this special oil hits a heated pan, you know you are doing something right
It's an instantly recognisable smell from when you are waiting, at the takeaway,for your curry to be cooked