As I am sure you all recall, motor oil base stock is categorized as follows:
- Group I and II - conventional (mineral) oil made from refined crude. Most high quality conventional oil today is Group II.
- Group III - Hydrocracked highly refined mineral oil that has many of the characteristics of true synthetics at a lower cost. In the US (but not in most of Europe) this can be sold as a "synthetic" motor oil, due to an FTC ruling about advertising claims. Not all Group III's are identical, as some are better than others.
- Group IV - Synthetic PAO
- Group V - Synthetic Esters (not necessarily better than Group IV in all cases).
In most of Europe, in order for an oil to be advertised as synthetic, it must not include any Groups I, II, or III base stocks. Therefore the base stocks must be 100% Group IV or V synthetics, These "true" synthetic oils are mandated by most German car oil specs, and such oils can withstand a longer oil change interval. When sold in the US, they are usually designated as "European Formula" oils right on the label because they are low in sulfated ash, phosphorus and sulfur (SAPS) as required by many European environmental regulations. SAPS additives do produce some extra pollution, but provides for better engine protection in most cases. However, because of this,
European Formula oils are not designed to be used in most American and Asian cars and will void your warranty since they are not API certified to meet the specs of those manufacturers (including Hyundai). Likewise, Amsoil Signature formula motor is not API certified (and will void your warranty) for US and Asian cars, even though it does last a long time. Amsoil makes two other lines that are API certified, with similar components to mass market oils.
One example of a European Formula Oil sold in the US is Mobil 1 0W-40 European Formula, which is designed for use in cars like MB. Although these European formula oils are low SAPS formulas, they do last a long time since they are true Group IV/V synthetics, and they are usually higher viscosity than is specified for American and Asian engines (which means that fuel mileage is lower with most European Formula oils).
Most synthetic motor oils sold today in the US (that are API certified for American and Asian car manufacturer specs) are either 100% Group III (Castrol Syntec, Pennzoil Platinum, Valvoline SynPower, etc), or a combination of Group III and Group IV/V base stocks (Mobil 1, Pennzoil Ultra, Castrol EDGE Extended Performance, etc). The above percentages do not include the relatively small percentage of oil additives that each contain.
A synthetic blend is not (as some may think) a combination of Group III and Group IV/V base stocks, but instead a combination of Group II conventional oil and a synthetic oil (usually Group III).
Personally, I always recommend synthetic oil. In general, any good quality synthetic "should" last 7500 miles, but with the advent of direct injection, etc, the oil does get dirty awfully fast, and is also subject to fuel dilution in the oil. Another option would be to use Mobil 1 EP (Extended Performance), Pennzoil Ultra, or Castrol EDGE Extended Performance, which should hold up longer, although not necessary deal with dirty oil question any better. Mobil 1 EP is only about $2 more expensive per 5-quart jug than regular Mobil 1 at Walmart, but may not be available at most dealers or oil change shops.