I must have messed it up a while back and it finally bit me when the upper petrol tank ran out:
Took me a while to figure out why plastic was working and not sulphur. I though it had something to do with when I offloaded excess light oil to a solid fuel array, but nope...then I noticed the windows on the two tanks (the top one mostly hidden by a large power pole) and had a closer look, which led to finally figuring out this little detail. At some point I unintentionally rotated the top storage tank, messing up the pipe connections.
Time to go back to using the Storage Tank Mk2 mod (the one with 8 connections that doesn't change anything when rotated).

Edit: No idea why the dropbox links won't display as IMGs, so I made them URLs.
Edit 2: Images fixed.