Each of these expressions is permissible. As if is used more often in speech and in informal writing than is as though, which is preferred by careful writers. Both expressions are followed by a verb in the subjunctive mood: "He left the room as though (as if) he were angry." In informal or substandard use, like sometimes substitutes for as if and as though but is never followed by the subjunctive mood. See also like.
Dictionary of problem words and expressions. Harry Shaw. 1975.