Engine oils have historically been mineral oils that have been refined from crude oils and Pennsylvania crude oils were often favoured for engine oil. Over the years, engine oil technology has improved dramatically and the base stocks used are now categorized into Groups. Groups I, II, and III are mineral oils that are progressively refined to a higher degree to eliminate impurities and varabilities in their molecular composition. Group I is solvent-refined oil commonly used until the 1980s but would now be considered a poor engine oil by itself. Group II oils are commonly used as conventional oils in modern vehicles today. Group III oils are considered to be synthetic oils now because of the severe amount of refining required to create them. Synthetic oils commonly found on the shelf at major retailers like Wal-Mart and Canadian Tire are generally Group III oils.
True synthetic oils are considered to be Group IV and V oils. Group IV oils are PAO (Polyalphaolefins) and Group V is everything else that is not mineral-based or PAO (such as POE - Polyolesters). Originally, Group IV oils were known for leakage due to their effect on seals but they no longer have this problem due to improved additives.
For multigrade oils, a conventional mineral oil (often called "dino" oil) starts off as a low viscosity oil (like a 5W or 10W) and polymer VII additives are added so that it performs like a 30 or 40 weight oil (ie, 5W-30 or 10W-40). In contrast, a synthetic multigrade oil is formulated as heavier weight oil (like SAE 30) and its molecular structure is manufactured so that it performs like a lower viscosity oil (like 5W) when cold. A synthetic oil accomplishes this with minimal or no use of VIIs.
Another advantage with synthetic oils is their high temperature stability. Although the oil temperature in the sump will generally not exceed 212°F (100°C), the temperature in the piston ring grooves can be much higher. Cars like the Saturn SL series (1.9L engine), for example, are prone to to carbon build-up in the rings. A synthetic HDEO oil will resist carbonizing in the ring grooves better than a conventional passenger car mineral oil and will prevent the engine from prematurely burning oil.