Oils and Diesel Fuels are broken down by numbering system, categorized by API Gravity (weight and viscosity), as well as BTUs.
#1 — #1 fuel oil, Kerosene, jet fuel, #1 Diesel, #1 heating oil
#2 — #2 Diesel, #2 Heating Oil
#3 — #3 Obsolete, no longer used, the ASTM no longer prints specs for it.
#4 — #4 Diesel, this is heavy oil used in some large marine
#5 — #5 fuel oil, also known as bunker oil, residuum, for boilers
#6 — #6 Heavy Oil, #3 heavy bunker oil for bunkers
Diesel or Diesel fuel, in general, is any fuel used in diesel engines. The most common is a specific fractional distillate of petroleum fuel oil, but alternatives that are not derived from petroleum, such as biodiesel, biomass to liquid (BTL) or gas to liquid (GTL) diesel, are increasingly being developed and adopted.
D6 is also known as residual fuel oil and is of high-viscosity. This particular fuel oil requires preheating to 220 – 260 degrees Fahrenheit. D6 is mostly used for generators.
Since it requires pre-heating, it can’t be used in small boats or cars. However large ships and power plants can use the residual fuel oil.
D2 is also known as gas oil. It is made from refinery Distillate #2. This type of fuel can be used without additives or reformers. D2 is primarily used as fuel in cars.
Jet Fuel (A, A1) Kerosene-type high-quality kerosene product used primarily as fuel for commercial turbojet and turboprop aircraft engines.
Biodiesel is an alternative fuel formulated exclusively for diesel engines; it’s made from vegetable oil or animal fats. Biodiesel can be mixed with petroleum diesel in any percentage, from 1 to 99, which is represented by a number following a B. For example, B5 is 5 percent biodiesel with 95 percent petroleum, B20 is 20 percent biodiesel with 80 percent petroleum, or B100 is 100 percent biodiesel, no petroleum.