The suggested oil for most current Nissan Altima models is SAE 5W-30 oil. Most standard consumer grade motor oils work for the Nissan Altima, but SAE 5W-30 is the suggested oil by the manufacturer.
To discover which oil is the best for a specific car, one can check the user manual that came with the vehicle. This also tells whether or not synthetics should be used. If the user manual is missing and a replacement can't be found, another option is to check on the oil cap itself. Many modern car models list the suggested oil type and viscosity right on the cap.