Adding Oil to your Dogs Nutrition.?

I read many of times that adding oil
(flaxseed, extra virgin olive, salmon, coconut, etc)
would help dogs (cats, horses, etc) to maintain their shiny coat & prevent it from shedding too much.
I feed Innova, which already contains flaxseed oil, so would it be bad if I add oil to his nutrition?


I was wondering WHICH oil is your choice and why?

Also HOW MUCH to you add and HOW OFTEN a week?

Thank you guys.