When dealership opens on Monday morning, call and ask to speak to the Service Manager. Let him know that his professionals put 0W-20 oil in your 2012 Sentra, and ask him if Nissan has back spec'ed the 0W-20 for your car. If he says "yes they did", ask him to email that information to you. You need to take no further action. If he says "no, they did not", tell him you will be by soon to see him and have the correct oil installed.