As both the recommended oil and the oil the dealer used are rated 5w, this means they offer the same performance in cold conditions. The difference between them is their performance at higher ambient temperatures, with 5w40 retaining its viscosity or 'thickness' up to ambient temperatures of 50C, while for 5w30 retains its performance at up to 25-30C. Strictly speaking we would always recommend sticking with the manufacturer's recommended oil specification, but the 5w30 has the advantage of potentially better fuel economy and better flow throughout the engine. Using 5w30 will not cause any short-term harm to your engine, but we would be tempted to switch back to 5w40 at the next service.