I thought I should ask the experts so I email Castrol Canada and got this reply:
"There is no shelf life for Castrol oil, however we would suggest that for
maximum protection and performance, the product be used within 5 years of
the date of manufacture. Regardless of the storage conditions, moisture can
get into the product via osmosis through the plastic.
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 C04001...., the "C" would represent the plant
location. The five digits '04001' represents the date of production. The
numbers 04 represent the last digits of the year (2004) and the '001'
represents the day of the year, based on 365 calendar days, on which it was
produced (January 1).
For C9001 '9001' represents the date of production. The number 9 represents
the last digit of the year (1999) and '001' represents the day of the year,
based on 365 calendar days, on which it was produced (January 1)."