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There are three big rules to remember when choosing engine oil: 1) Synthetic! Natural Oil, Dino Oil, Conventional Oil, or whatever its called is a bad decision. Natural oil comes out of the refinery at a 40 to 60 weight oil. To bring the weight down the oil is thinned out with paraffins, waxes, kerosene, paint thinner and who knows what else. These additives bring the weight of the oil down to 20 or 30 weight. None of these additives lubricate the engine. Only 60% of the fluid in a quart of natural oil is really oil. And that oil is a heavy 40 to 60 weight. The remaining 40% doesn't lubricate. Most engines don't get hotter than 220 degrees fahrenheit. Should an engine reach 230 degrees fahrenheit the additives in the natural oil evaporate. And the 30 weight oil becomes a 60 weight oil. The vaporized oil is pulled through the positive crankcase ventilation (PCV) and into the intake manifold and fouls up sensors like the manifold air pressure (MAP) sensor. The natural oil does not lubricate well. It withstands 300 pounds per square inch before it gets forcefully shoved aside. Although that may sound like a lot of pressure the oil cannot successfully lubricate a cylinder under the pressure of combustion. Heavy weight oils can be a parasitic loss. More friction and less heat dissipation don't help engine efficiency. The only time a heavy oil is useful is for worn out engines that need the oil to fill the gaps of worn clearances between parts.
The photos above show three exhaust rocker arms from a Chrysler 2.0L sohc. All of three of these rockers came from the same engine. The engine lived a life on Penzoil's natural oil from the previous owner. This left the engine parts coated in a nasty black layer of burned oil. Valvoline's natural oil replaced the Penzoil natural oil. The Valvoline cleaned the parts enough to leave a bronze coating of oil on the surface. Mobil1 synthetic was the last oil used. The synthetic oil effectively cleaned the engine parts. Synthetic Oils Synthetic oil withstands 8,000 lbs of pressure per square inch. That's more than 26 times as strong as natural oil. Synthetic oil can be manufactured to an exact specification with consistency batch to batch. At least 95% of the oil in a quart of synthetic oil is lubricating oil. The molecules are much much smaller. Synthetic oil is not more slippery like some manufacturers may advertise. And not all synthetic oils are created equal. Mobil1, Valvoline, and Pennzoil have the resources to ensure a consistent product batch after batch. The oil that doesn't meet spec gets repackaged and sold at places like Walmart. Mobil1 offers a 0W-30 Advanced Fuel Economy engine oil that advertises a 2% increase in gas mileage. And they advertise savings of $147 per 50,000 miles for a car that gets 20mpg on $3 per gallon gasoline. It's a small amount that is difficult to measure. So a difference like this may not be effectively challenged. But 2% here, 3% there, and 5% from who knows what else can show a 10% increase in gas mileage pretty quickly. A zero weight (0W) is equivalent to a 5W weight with the exception that the 0W weight can give a uniform performance in colder temperatures than 5w weight oil. Not all synthetic oil is created equal. STP oil has zero lubricating ability. The 20D's of added protection on the label are really 20 drops of natural oil. This is how they get around the Federal Trade Commission. But it's still false advertising. Slick50 contains PTFE (Teflon). DuPont, the chemical company that created Teflon (PTFE), said Teflon is not a useful ingredient in oils or oil additives used for internal combustion engines. The Teflon builds up in non-moving parts like a clot in a an artery. When the Teflon burns at the engine's operating temperature the stuff melts together making the clots larger. Teflon is a solid that can behave like a lubricated surface in certain conditions, however, the internal combustion engine isn't one of those conditions. Extended drain intervals shouldn't exceed 6,000 miles. Even if the manufacturer says the drain intervals can be extended to 15,000 to 25,000 miles, change the oil at 6,000 miles anyway. And the oil filter should be changed every 3,000 miles. If you're on a budget, mixing synthetic with natural oil would be okay. A 25% mix of synthetic oil to 75% natural oil would be as thin as I would suggest diluting it. A 50/50 mix would be even better.
What have we learned? Synthetic! Drain intervals should not exceed 6,000 miles. And the recommended brands are Mobil1, Valvoline, Pennzoil, Castrol, and Amsoil.
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