The myth that you shouldn't switch back to dino after using synthetic has largely been debunked. But is there any issue with switching frequently between synthetic and dino oil? And would using synthetic only every 3rd or 4th oil change be worth it, or is it not worth it unless you use synthetic exclusively. These are hypothetical questions, but came about because I can change the oil in my cousin's Lexus every now and then with my stock of Mobil 1. But it usually gets Valvoline dino oil changes at the dealer.