Sac oil does not have a powerful odor like castor, but the beaver can smell it. They have a very good nose. But of the two, castor will generally provide the greater attraction for beaver. Castor is a territorial marking scent and enlists a challenge response from the beaver. Sac oil, is a body odor and does not elicit the same strong response. Personally, I use sac oil only for special sets where I am trying to catch an educated beaver that may not be responding to a castor based lure. And I only use a couple of drops at the set. Again, don't worry about them being able to smell it -- they will.
-- Hal