Different fuel grades

Many modern refineries include treating fuel to reduce the amount of pollution generated from combusion (hydrodesulfurization/nitrification/chlorination, etc.) as well as increase the amount of useable energy by allowing said fuel to be used in higher performance engines. Adding different fuel grades (Bunker-C/HSFO for cargo ships and boilers, low sulfur to ultra-low sulfur diesel for trucks) will further incentivize upgrading oil refinery capabilities and complexity while giving the player another avenue for reducing air pollution and increasing sulfur/ammonia production.

It had occurred to me before as well that in reality there's different fuel grades. I think in the game the easiest way to implement this would be to allow cargo ships to use heavy oil as a fuel, but using it makes their trip time much slower. It makes a lot of sense because that's exactly what cargo ships do in real life, and it would provide an alternative usage for heavy oil, since right now you cant use it directly for anything.