cummins isx 2009 international prostar premium

[seanmarl]

Hello,
while doing the soot cleanout of the intake/egr system etc. after pulling the manifold I was looking in the head with a flashlight and around I believe the #4 intake valve (about dead center of the intake port in the head) it is wet with what looks like oil. Is this most likely valve guides? or possibly a leaking injector? maybe when I get it back together I was thinking of doing the injector leak test as im thinking if the injector is leaking this is where it would be?
thanks and sorry if my logic doesn't follow, im pretty new to these engines.