I don't know if everyone already knows this, but it looks like Mobil1 puts the packing date(?) on their quarts of oil in ink that is only (easily) visible under UV light. Here's a picture I took with my UV lamp. The printing reads "06/08/04 20:09 LN8 1142". I just saw "National Treasure" and I was looking for invisible writing around the house and I stumbled on this. I'm bringing my UV lamp next time I buy oil.
