Yes. It depends how your respond to it. A decent dose of primo is more powerful than syntheselen, but synthelsen is still effective and won't shut you down like an AAS would. I would class it as a performance enhancer, rather than comparing it to an AAS, per say. But a cruise between cycles would be a great time to run it.
Whether you add lean muscle on it depends on your diet and how much lean muscle you already have, plus how far past your genetic limit you are.