The only definite way to distinguish between mineral and synthetic oils is in the laboratory. Laboratory tests can distinguish synthetics from mineral oil by looking at a combination of physical properties including VI, flash point, pour point, and aniline point. There also may be different elemental chemistry.
Unfortunately, color is not a way to distinguish between the two types of lubricants. Because mineral base oils are less refined than synthetic base oils, mineral base oils contain more aromatics, sulfur, and other impurities. These impurities tend to give mineral oils a darker natural color than synthetic base oils, which are nearly transparent. Though this may be a distinguishing factor for base oils, additives can considerably darken the motor oil once they are incorporated into the finished product. Therefore, unfortunately, there is no reliable way of differentiating between mineral and synthetic motor oils in the field.
One prime example is the VX-12 additive technology included in TOP 1 Oil HP Plus. VX-12 utilizes tri-moly molecules, which are metallic and very dark. Therefore, even though HP Plus uses a synthetic-blend base oil that is nearly transparent, the VX-12 additive package gives the finished HP-Plus product a dark color.
The final way to distinguish between mineral and synthetic oils is to put them to the test of time. Over any time interval, synthetic lubricants will last longer, keep the engine cleaner, and prevent the engine from corrosion better than mineral lubricants. Of course, the differences will be more pronounced the longer the test interval.
In conclusion, though you may not be able to visibly distinguish mineral oils from synthetic oils, your engine always will be able to.
