Otto,
If you note on the GMC valve cover, there is no "oil filler" provision.
The removable cap on the draft tube (correct me if I'm wrong GMC guru's) is for filling ye 'ole crankcase with fine motor oil.
As to how the underside of that cap looks, I can't say as my x-ray vision stopped working decades ago - haha!
How about it GMC guys??? What's under that cap? My best guess is just a simple baffle plate a bit down the throat of the tube???
Dan
