oil filter empty after 5K miles!?!? Why?

[azex2001]

1991, EFI, 5sp. 409K miles. 10w-40. STP oil filter. I changed the oil this weekend and when I removed the oil filter it was empty (no dirty oil) and clean. I always put clean oil in the filters before installing them, so I could see that it had some oil in it, but it was empty and clean. Any ideas what happened? Thanks.