Regarding common "synthetic" oils, such as Mobil 1, there is not a significant molecular difference between the base of the "synthetic" and the base of regular "cheap" oil, expect that the "synthetic" has more wax filtered out thanks to advances in crude oil processing. Premium Class III motor oils labeled as "sythetic" have better detergents and additives, along with undiscosed amounts of actual synthetic material mixed into the base-stock. Off-the-shelf "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. 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. A "false seal" is the condition in-which sludge eats away at a seal, but doesn't actually leak because the sludge itself acts as a seal. If you have false seals, then a PAO based oil will displace the sludge and leak. The better designed engines and oils of today are less likely to create sludge due to the fact that modern engine are less likely to burn oil, combined with modern oils that are more difficult to break-down. The engines and oils of yesterday created sludge, and when PAO based oils were used would leak. This is how the rumors of "synthetic-oils-leak" began.
All modern PAO oils also have additives that fully lubricate and seal all rubber seals. I switched to a PAO oil with no ill effects in my Gen 4 4Runner when I bought it with 100,000+ miles. If you're switching to a name brand "synthetic", then don't worry about a thing. If you're switching to a true synthetic, then don't worry about a thing.
The most common reason for people to use PAO oils is for extended drain intervals. The true chemical benefit Class IV offer to engines is the oil's ability to stay pourable at extremely low 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, more additives are used. Through the life of an oil, additives are going to deteriorate much quicker than the base-stock. PAO oils are less depended on additives to maintain the cold weather weight, and a little less dependent on buffers to neutralize acids. Hence, PAO oils have long life.
