Common "synthetic" oils, such as Mobil 1, use the same chemical base as conventional "non-synthetic" oil, expect that the "synthetic" has more wax filtered out thanks to advances in crude oil processing. Class III motor oils labeled as "synthetic" have better detergents and additives, along with undisclosed amounts of actual synthetic material mixed into the base-stock. Off-the-shelf, Class III "synthetic" oil is better than regular motor oil as it is more refined and has better additives, but the term synthetic is a marketing ploy. Class III motor oil is highly refined crude oil. The term "synthetic," when it comes to motor oils, is a marketing discription and not a chemical differentiation.
PAO oils, falling under Class IV, are fully synthetic and are extremely uniform at the molecular level. The two most common reasons for the use of PAO oils is for extended drain intervals, or to combat extreme weather. The true chemical benefit Class IV offers to engines is the oil's ability to stay pourable at extremely low temps (without the need of additives to keep the oil thick at operating temps), and to displace acids and burnt residues. For a Class III oil to come close to a Class IV oil's cold weather viscosity (pourable at cold temps, yet remain thick at operating temperatures), more additives are used. Through the life of an oil, additives are going to deteriorate much quicker than the actual base-stock of the oil, whether the base stock is PAO or some form of crude-derived oil. PAO oils are less dependent on additives to maintain the cold weather weight, and a little less dependent on buffers to neutralize acids. Hence, PAO oils have longer life.
Mobil 1 is a great oil. I live in an extreme environment, so I've opted for a PAO based oil: Amsoil's signature series. I'd have no problem using a Class III if needed.
Whether or not to switch is up to you, as both oils are great. I'm not a fan of marketing descriptions, but the oils discussed here are both great.
