I recently purchased a 1955 No. 12 motor grader (80C series) that I'm refurbishing.  The grader has the D318 engine but has a pipe connection from what would be the fuel injection pump filler to the engine crankcase.  I’ve never seen such a connection and frankly, don’t know why there is one.
How is the fuel injection pump lubricated?
   

Check the 80Cxxxx parts book pages 32 & 33.
Oil is supplied from a line coming from the engine lubricating system to the governor with drain back through the injection pump and from there back into the main engine sump.

Thanks Old Magnet; I was able to find the illustration in the parts book on the pages you sited.  It looks like a better way to lubricate the governor and injection pump than the traditional sump process..
It curious because I have a D6 9U that was one of the last 9Us made in 1959 and it does not have the governor/injection pump lubrication plumbing that my 1955 grader D318 has.  Was this lubrication configuration unique to the D318 graders?

Seems to be a very late addition. I could only find that configuration on the 80C and the #14 78E Graders. The 78E was the last of the D318's, was factory turboed at about 150 hp. Only 127 units built.
Thought it might have been used on the late D318's "G" Series  45Bxxxx Industrial Engines but not so.
Edit
Add the 70D and 71D to the list. Seems to be a grader thing only.