Yep, the answers here are pretty right on and sum up the advantages of synthetic media.
-better efficiency with less pressure drop
-better oil flow pre square inch of media at any efficiency equivalent
-higher contaminant capacity per square inch of media
-vastly less susceptible to media deterioration from water & acids
Downsides?
-no ability to absorb moisture (this ability is a two edged sword, but it's generally advantageous to grab small amounts of water from the oil and evaporate it before it has a chance to start the various chemical reactions in the oil)
-fractionally less able to capture certain types of oxidation residue
-higher cost
Yes, Donald, syn media is sometimes (if not often, though I have not checked in detail on each syn filter offering) made up of multiple layers of media of varying density and the outer is less dense. It's also possible to manufacture media in varying densities. Since the fibers are "engineered" they can be made in any size and shape to promote flow filtration. Synthetic nanofibers are fast becoming a part of the cellulose world where they are added to the media to improve efficiency without much cost in the flow/DP departments.
As to nomenclature, the "synthetic" name seems appropriate, though it is used rather deviously in marketing. Cellulose is manufactured from "natural" products. Synthetic media is man-made/engineered. The ad pukes know that "synthetic" reads as "better" to most folks, especially low information oil consumers. Still, that doesn't change the facts that they are indeed "synthetic" media filters and they are "better" in some measurable ways.
To save Dave Newton the trouble of chiming in ( : < ) whether that "betterness" is appropriate or cost effective in all cases is another question and that's where the low information oil consumer gets tripped up by the ad hype.
