I have a 2003 Malibu LS with 87,000 miles on it. The motor runs very smooth, it's getting excellent gas mileage, and etc. It's not leaking, burning oil, or smoking. I currently use Castrol GTX high mileage 5w-30 and its great, but I want to get the best out of my motor and make it last longer. I was thinking about switching to Castrol Syntec which is full synthetic. I want to switch because of all the benefits. Castrol, Pepboys, a chevy mechanic, and a mopar mechanic said yes switch, but my local dealership said no cause the motor has too many miles on it and could start leaking, burning oil, and smoking? anyone think it's safe?