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Several readers have asked for more details regarding the differences between conventional and synthetic motor oils and whether the latter allows extended change intervals.
There are tens of thousands of posts and forums on the Internet regarding this issue with that many and more opinions.
I recommend vehicle owners follow the maintenance schedules and use the oils recommended by the vehicle manufacturer, over those of an oil company or aftermarket supplier.
The engineers at the car company have conducted wear, fuel efficiency and emissions-related tests on the engines they designed and developed. They establish warranty parameters and list recommended lubricants, change intervals and descriptions of driving conditions and habits.
Those recommendations, almost without exception, contain two key specifications – viscosity (5W-20 for example) and API service grade (SJ, for example).
Even a quick peek at the label of an oil container reveals claims of meeting a myriad of other standards. But most of them are irrelevant for North America roads and driving.
Having said that, here is a little primer on engine "synthetic" and conventional engine oil.
It is (falsely) believed that "synthetic" engine oils are made entirely of synthetic materials starting at the molecular level, while conventional oils are derived from crude oil.
Back in the late 1990s, Mobil Oil filed a lawsuit against Castrol claiming the latter was marketing oil under the "synthetic" label that contained some petroleum-based stock. Mobil lost the suit and it was decided the word "synthetic" was a marketing term and referred to properties, not to production methods or ingredients. Castrol Syntec, as "currently formulated," was declared a "synthetic motor oil."
Both the Society of Automotive Engineers (SAE) and the American Petroleum Institute (API) subsequently revised their definition of engine oils to allow a wider interpretation of what constitutes "synthetic" oil. From that point, most "synthetic" oils contain some degree of what is known in the industry as Group III base stock, a highly refined petroleum product.
The result is a product with nearly the same properties as "true synthetics," at a lower cost. Pure synthetic oil is available – read the labels closely.
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