Let g be a complex simple Lie algebra and e a nilpotent element of g. We are interested in the isomorphism question (raised by Premet) between the finite W-algebras constructed from some nilpotent subalgebras of g called e-admissible. We introduce the concept of e-admissible pairs and e-admissible gradings. We show that the W-algebra associated to an e-admissible pair admits similar properties to the ones introduced by Gan and Ginzburg. Moreover, we define an equivalence relation on the set of admissible pairs and we show that if two admissible pairs are equivalent, it follows that the associated W-algebras are isomorphic. By introducing the notion of connectivity of admissible gradings, we reduce the isomorphism question to the study of the equivalence of admissible pairs for a fixed admissible grading. This allows us to prove that admissible pairs relative to b-optimal gradings are equivalent, hence the corresponding W algebras are isomorphic. We recover as a special case a result of Brundan and Goodwin. In the final part, we use our results to find a complete answer to the isomorphism question in some particular cases.