For any specific grease you want to compare you should get the data sheet for the grease from the manufacturer, which is often easily available from the mfr's web site. (Not the MSDS, the Product Data Sheet). There you will find the actual penetration test value. Compare to this chart:
http://www.greasedispensing.com/ref-viscosity.htmland you can tell whether it is a "light #2" or a "Heavy #2", etc. This same information is also available on wikipedia or directly from NLGI.
As someone else posted, this is only viscosity information, not to be confused with lubricity, high temp. performance, etc.
Ken