I'm replacing my underdrive pulley with my stock pulley. I just pulled the crank bolt out and it had oil on the threads of the crank bolt. Where would this oil have come from? Is it anything I should be concerned with?

Truck is a 2005 2500HD with 6.0L & 70k miles.

 

Don't quote me on this it's just a shot in the dark buty guess would be the seal inside the timing chain cover that surrounds the crankshaft... Did your truck ever have any oil leaks underneath especially after driving for a while?