You need to realize that just because the arrow points to a month, the tail of the arrow isn't pointing to a year. Using your logic, the stamp would only work for one month per year.

When a date code insert stamp is used to show month and year, and it is a common practice, then the year is often stamped separately. See the photo for an example. The centre piece is replaced each year, and it is rotated each month. If the year isn't important, then the centre piece only has an arrow on it.

Also, the stamp is used for lot traceability of the bottle manufacturing process, much more than for the fill date. Fill dates are often printed on, as the date stamp you are looking at is created at the time the bottle is molded.

Now, just use the oil already.

Attached Images

 

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2007 X3 3.0si, 6 MT, Premium, White

Retired:
2008 535i, 6 MT, M Sport, Premium, Space Grey
2003 X5 3.0 Steptronic, Premium, Titanium Silver
2002 325xi 5 MT, Steel Grey
2004 Z4 3.0 Premium, Sport, SMG, Maldives Blue