Conventional, Synthetic & Blends

Oil can be broken down into three different categories, conventional (or mineral), synthetic and synthetic blends.  In short:

Conventional/Mineral: Oils created by extracting materials.  These oils are a mix of different size molecules.
Synthetic:  Man made oil created by adding materials.  The molecules of these oils are uniform in size and shape.

Blends: Blends of both synthetic and conventional oils.

While Synthetic and conventional oils might appear the same to the naked eye, they are different at the molecular level.

In general, synthetics provide low pour points, high viscosity index and better lubricity than conventionals.  They tend to perform better at high temperatures, last longer and generate less waste.

HOWEVER, this does not mean synthetics are the correct choice for all applications.  Most synthetics are NOT compatible with other oils, be they synthetic, conventional or blends.  Mixing synthetics with these can cause serious engine damage.  Synthetics are also a poor choice for wet conditions, as they will absorb large amounts of water quickly and create acid.  Finally, synthetics have such powerful cleaning properties that they can remove paint or even dissolve seals.