Dear Selvi
This is how I use:
Ex V olive oil for all shallow fry veggies, kuzhambu thaluchify etc.
Premium sunflower oil for depp fry vadai, appalam,vadam
Nallennai (Idhyam)- for vathakuzhambu,kalandha saadams.
Coconut oil for avials, thenga saadam.
As many ILites suggested, I read an article recently that the oils should be mixed for balancing the cholesterol levels in the body. Also it pointed that oils should be used appropriately according to their level of smoking points. I will give an extract of the article soon.
Happy cooking.
Cheers
Vijee