Would Blown Plenum Gasket cause external oil leak?
It seems to be leaking onto the oil filter.

 

No, plenum is internal but a leaking valve cover or a bad filter mating surface can cause a drip to appear there as well as few other things. Get a drop light, get underneath and on top and have a look. Plenty of room to see and you should spot it.

 

x2 on valve cover leak, it will drench the back half of the block in due time

 

I think I can see where it is leaking from its just a few inches above the oil filter, but I'm not familiar with what part it would be, It looks like its further down than the valve cover. It would be a hard pic to take but I will try and take a pic..

 

Will either of these help?

 

Yea that does help I think it's leaking from the corner that I circled, or its just leaking onto that spot, but thats where it looks like its coming from to me.

 

Reach around the back side of the heads and see if you can feel oil it can draw to that area with a leak from the valve covers or intake, if you can feel oil back there keep reaching till its dry.