The main difference between synthetic motor oil and conventional motor oil is found in their molecular structure. In a mineral oil, the molecules come from organic, natural materials, and as we know, nature isn't always consistent. There can sometimes be a few oddball molecules in mineral oils. Synthetic oils, on the other hand, were created by scientists in a lab. The molecules are uniform, and they line up like good soldiers inside of your engine.
There are three basic parts to synthetic motor oil:
- · The base oil
- · Performance additives, which come in powder form
- · The carrier oil that disperses those powdered performance additives throughout the base oil
There are a few terms that come up with synthetic oils that you should know:
1. Viscosity Index: This measures the effect of temperature on oil viscosity, or the oil's thickness and ability to protect the engine. When oil is heated, it becomes thinner; when it's cooled, it becomes thicker. If the oil is too thin or too thick, the oil can't do its job properly. A high viscosity index means the oil doesn't change too much, no matter what the temperature inside the engine may be. In a perfect world, the viscosity of the oil wouldn't change at all and would provide optimum protection under any conditions.
2. Total Base Number: The "base" in this context is the opposite of "acid." The total base number measures the oil's ability to withstand acid build-up in the engine.
3. NOACK Volatility Number: Volatile compounds are unstable and tend to vaporize when exposed to heat, and this test measures that tendency. As temperatures rise, smaller molecules vaporize, leaving behind larger molecules that can make oil more sluggish and less viscous. The lower the NOACK volatility number, the better; it means there are fewer molecules being lost, which means fewer top-off's at the local lube shop.
In all of these tests, synthetic oils perform better than their mineral oil counterparts, thanks to those uniform molecules. Sure, lab testing's great, but how do these oils perform in the real world? Let's explore more!
source: how stuff works, wiki