As I mentioned it already, I mix 10-20g (1-2%) rubbed Oregano under 1 kg feed. I buy it in 1 kg bags. Additionally I mix 1ml (per kg feed) Oregano feed oil (I use the one with 2% for pigeons) under the feed - as binder for the rubbed Oregano.
Here are many ready to use oils and liquids, which are based on Oregano.

As far as I know, the 2% and 10% oils are blended with Soy oil.
I don't know, whether it is GM-free Soy oil.