That is not true. Every tanker of oil is tracked. LLoyds of London sells the info for trader information.
Also, each field has some unique measurable properties.
The above are only the most basic. When an Assay of Crude oil is done, the measurements may include:
API gravity
Cetane index
Cloud point
Cold filter plugging point
Density
Flash point
Freeze point
Hydrogen Sulphide %
K factor ( paraffinic, naphthenic, intermediate or aromatic)
Mercaptans
PONA number
Pour point
Ramsbottom Carbon Residue
Reid vapor pressure
SARA (Saturate, Aromatic, Resin and Asphaltene) Smoke point
True vapor pressure
Viscosity
% of heavy metals
Each crude oil type has unique molecular, chemical characteristics. No crude oil type is identical and there are crucial differences in crude oil quality. The results of crude oil assay testing provide extensive detailed hydrocarbon analysis data for refiners, oil traders and producers. Assay data help refineries determine if a crude oil feedstock is compatible for a particular petroleum refinery or if the crude oil could cause yield, quality, production, environmental and other problems.
As an example:
http://www.bp.com/extendedsectiongenericarticle.do?categoryId=9011898&contentId=7022393