Manufacturer Oil

Hey guys I have a question for you all. When I purchased my 2010 Frontier, the dealership offered free oil change and inspection for life. I asked them what motor oil they used and they informed me they used synthetic blend 5w30 Pennzoil. I called two other local dealerships who said they used full synthetic Mobil 1 and another synthetic blend Havoline. The manual on the truck specifies 5w30 but does not mention anything about conventional or synthetic. I called the Nissan plant where the truck was built and they said they used Shell conventional in all the new trucks. What have you guys heard and for those of you who have it done at the dealership what brand/kind do they use. Thanks in advance for any feedback.