The performance of synthetic oils should be better than straight refinery oils, as the base oil in synthetic oils is chemically manufactured, which gives it better properties than straight refined oils, due to the ability in the synthetic manufacturing process to produce purer base molecular structures, with the most desired oil properties.
Oil refined directly from crude comes with a range of chemical component structures that are not necessarily ideal for lube oil use, and the oil makeup has to be balanced by adding other additives to counter the undesirable chemicals.
Despite clever refining processes, the undesirable chemicals still slip through in the final base oil refinery product, because quite of number of the undesirable chemicals are either expensive to refine out, or have chemical properties that enable them to slip through hydrotreating.
The bottom line is all base oils have at least 7 extra additives added to them, to make them perform to the required level or oil standard. These additives are often quite different between the various manufacturers, and the type/selection of additives would more than likely play the biggest part in an oils performance.
The basic additives are;
Antioxidants to prevent oil breakdown by oxidisation
Acidity neutralisers
Rust and corrosion inhibitors
Viscosity index improvers (VIII's) to maintain oil thickness across a range of temperatures
Anti-wear agents
Detergents and dispersants to keep internal engine components clean
Anti-foaming agents
Extreme pressure (EP) additives for heavy-duty applications
The oil chemists have a huge array of chemical additives to choose from, all with different properties and performance - and they keep finding new additives all the time.
Plus, the chemists have to balance oil performance against their manufacturing costs, so if you want exotic, high performance additives, you have to pay.
In general, the oil film will drain off metal surfaces it's supposed to be protecting, within as little as 3 months, and chemicals in the oil either settle, react slowly with each other, or simply degrade by chemical bonding breakdown, over an extended period. Therefore, using fresh oil, and keeping surfaces coated with oil, are two of the most important things you can do, to prevent engine problems related to poor oil performance.