Subconscious and unconscious are synonyms when they’re informal adjectives meaning occurring in the absence of awareness or thought. For example, to say that kittens make you feel anxious on a subconscious level is the same as saying they make you feel anxious on an unconscious level. But unconscious is the more scientific term, and it’s the usual choice in science and medicine. Subconscious is fairly common in quasi-scientific writing, but its definition is fuzzy, and it often signals that the writer lacks real expertise.
Of course, where the adjective means lacking consciousness, unconscious is always the appropriate word.
In these examples, subconscious and unconscious are more or less interchangeable:
Such names, he said, speak to a subconscious, social trend to raise boys as gentler beings. [Edmonton Journal]
Many young people today grasp that, at least on an unconscious level. [Los Angeles Times]
He plunders the powers of the subconscious to reveal clever little tricks you can use. [Daily Mail]
What you’re doing with this technique is working with the unconscious mind, which is extraordinarily powerful and sometimes very stupid. [Grand Junction Free Press (link now dead)]
And in these instances, subconscious would not work in place of unconscious: