The first car is my own, a 2008 Ford Fusion 2.3L. 101k miles as a former rental car. Currently I take it on 20 minute long commutes in moderate stop-and-go traffic almost every day, highway usage is uncommon with this car. The second car is a 2004 Nissan Altima 2.5 S. 124K miles under its belt. My sister takes it on a commute almost 45 minutes long on a mixed city and highway drive and a 200+ mile road trip about once a month. Both cars have no leaks, and I've not used a synthetic for either one before. Respectively they get filled with Motorcraft 5W-20 and Valvoline NG Conventional on 5k OCIs. Would now be a good time to switch to a full synthetic way before the impending summer heat wave rolls around?