The biggest thing that sold me on synthetic was the first time I put Mobil 1 in my 92 Jeep - about 4 years ago now I believe.

140k at the time, always had conventional. Had a heavy tap all the time, which I know is a stuck lifter. The first time I put Mobil 1 in was for the winter, 5w-30, to try it out. Yes, 5w30 in an engine with 150k - it was synthetic, I heard it would be fine.

Not only did it start easier than it ever did, I had half as much blowby coming out the CCV system in 500 miles, and the tap was gone after about 200 miles. Now I only ever hear it when it's cold and I'm straining it, like going up a hill at 50MPH in overdrive.

All of my cars have synthetic now. The Jeep has 172K, the 300M has 162K, my Charger has 44K so it should be fine of course. My mom's 300M has 101K. None burn any oil visibly, and although the Jeep has blowby it uses less than a quart per change on 10w40 (a quart on 5w30). My 300M with 162K doesn't use anything between changes, it's still on the full line when I change the oil at 4500-5500 miles.