The problem with silicones is that you need a SULFATE to wash them out.
You definitely don't need sulfates to wash out oils, you can do it with mild cleansers.
And that is why oils are CG friendly, and why silicones are not.
Oils are very different from silicones. Oils get absorbed by the hair. But it depends how much you use, not everything gets absorbed. And it depends on the oil. Oils are partially water soluble... co-wash removes most of the build up but not everything. Jojoba and castor are the least water soluble I believe. So yeah, oils can build up a little, especially if you have fine hair. So it is important to do a mild clarifying wash every now and then, I use Nature's Gate ( http://www.iherb.com/Nature-s-Gate-R...oz-532-ml/6165
) for that. It is a sulfate free mild shampoo and works excellent for me when I want to remove oil build up.