You normally don't need to plug those as they are just accessory mounts which are on both ends of the heads so they can be installed in either direction.

Probably just left over oil from the machining process.

 

If you already have the engine installed and running, oil may be leaking from the valve cover gasket, and just appears to be running out of the accessory holes. If the engine is still on the pallet, then forget this post and listen to previous guy!

 

Aren't those accessory holes? If so I'd be shocked if oil was coming from there. As stated above, check the valve covers (and those gaskets...)