I don't know if everyone already knows this, but it looks like Mobil 1 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.