My wife has a 2015 Nissan Altima that she got new. It has a little over 30,000 miles on it. The recommended oil is 0W/20 but the Owner's Manual says that 5W/30 is an acceptable substitute. I've been changing the oil and filter every 3,000 miles since new using Valvoline 5W/30 conventional oil. I can only find 0W/20 in a synthetic. I don't care that the synthetic costs more. Should I switch over to the 0W/20 synthetic? We typically keep our vehicles a long time.