My tweak would be to move the fuel availability over one grade, like so:
A-class ports: Refined fuel only
B-class ports: both Refined and Unrefined fuel
C-class ports: both Refined and Unrefined fuel
D-class ports: Unrefined fuel only
E-class ports: Unrefined fuel only
X-class ports: no fuel available
Also, I would assume that D-class ports have minor repairs and maintenance, while E-class ports have a port control office and maintenance facilities, but no repair facilities; maintenance might just mean having some leftover parts and a few tool kits, of course.