Another method I've used with success is talcum powder. You can put some in a squeeze bottle and dust the area (once it is either cleaned or at least wiped off). Drive the truck around the block and any leak will be immediately apparent as the dry powder soaks up the oil.

I had a sand hole in the casting of the engine block (under the motor mount!) on a 1990 Jeep Cherokee 4.0L, and without the powder, I don't think I would have ever found the leak. Good luck.