WD-40 is about 1/3 petroleum OIL. Granted it is a light oil, but an
oil nontheless.

WD-40 is primarily Naphtha a Paraffin.
http://www.wd40.com.au/msds/ChemWatc...-40%20Bulk.pdf

The Naphtha disappears quickly as it removes
the legitimate lubricant from the item that is in
need of lubrication. It then leaves a coat of
Paraffin behind.

See link:
http://www.lube-tips.com/focus/2006_02_15.htm