Simon.
Luboil reducing valve.
I guess the pipe is a manometerpipe. Below is the system, there are two small pipe connections on the reducing valve, the large is the returnpipe to the system oiltank joining the overflowpipe from the luboil cooler and the luboilpump (yellow). The smaller pipe must be a manometerpipe, you want definitely to know that this important valve works with the correct pressure. There is a manometer at the end of the engine as well.
Tore