Your Owners Manual will have the proper weight and viscosity requirements for your Sentra. The manual calls for 5W-30 API Certified oil. In higher-mileage engines, you could also go with a 10W-30 API oil. (capacity = 3.38 quarts.) Brand is not important, just make sure the oil has API Certification (usu. marked on the back label). Mixing synthetic with conventional oil is fine, at any ratio, but never mix oils having different weight or viscosity.
Your Sentra does not require synthetic oil, but you can use it if you wish. I like Mobil1 synthetic which is readily available. Use 5w30 in either conventional or synthetic and use a name brand. A high mileage oil would be fine also.
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