I sent them an e-mail asking how to decipher their batch numbers.
Here's the response, revised by me a little to make it more readable.
"Thank you for contacting Castrol North America.
You can determine the date of manufacture by locating the fill code on the bottom of the back panel.
It is composed of a series of letters and numbers, with the first five characters being the most important.
For example, if you have M04023...., the "M" would represent the plant location.
The five digits '04023' represent the date of production.
The number 04 represents the last digit of the year (2004) and the '023' represents the day of the year, based on 365 calendar days, on which it was produced (January 23).
Therefore Lot# M040236ATW2418346 16:38
was manufactured on January 23rd of 2004."