Add one to the synthetic oil myth list. It goes someting like this:
1. Your rings won't seat properly if you use synthetic in a new engine...(that's why Porsche, M-B, GM, Farrerri and others use it from the factory).
2. If you have a roller cam it will ruin the roller bearings in the lifters due to them not turning because of the reduced friction. Same with a Harley-Davidson motorcycle engine that uses roller crank bearings. (the friction coeficient is the same as dino).
3. It will find it's way out of the same spot dino won't because it is 'slicker'...it won't if it's the same weight oil.
What it will do is clean all the deposits from the gaskets (assuming you were not using Castrol-GTX) and leak like a screen door due to the gaskets having shrunk.(especially if you were using an oil with moly-dendum additives)
IMHO unless you are running a late model car that generates the kind of heat these new cars do you should just run Castrol-GTX 20-50 and be done with it.
It won't make you engines water temp drop since this is controled by the thermostat and the cooling system and not by the oil. I've run it in an 87 Taurus, an 87 Sable, a 1982 M-B 240-D, a 1979 Fiesta (starting at 100,000 miles with no bad side effects), a 1995 Z/28, 2000 Civic, 2000 Accord, 2007 Accord, 1977 XLCH, and my 2004 BMW r-1150-RT. Absolutely NO difference in either water or oil temps and the bikes are air cooled with the XLCH being all cast iron above the cases.
I'm positive you will see posts here telling me I don't know what I'm talking about, but I've been doing this since the late 1950's and I will tell you the truth....even when it's something you may not want to hear.
Don't waste your money on synthetic in an older car.
And the guy who saw a 2mpg increase in milage....was it the same weight oil as dino or did you use a 5-30 energy conserving oil when you changed to synthetic and what was the viscosity of the dino? If you use a 5-20 E/C oil it should not matter if it's dino or synthetic. The thickness of the oil is what determines how much work it takes ot push it through the engine once it's warm.