Stop Calling It D6. Here’s What It’s Actually Called.

Picture a procurement manager at a midsize refinery, somewhere in Houston, Rotterdam, or Singapore, opening his inbox on a Tuesday morning. He’s already had two coffees. He’s not in the mood.

The first five emails use the phrase “D6 Virgin Fuel Oil” or ask whether the refinery can supply “D6 at CIF price with SBLC.” He deletes them without reading the rest. Not because he doesn’t sell what they’re asking about. He does. He deletes them because he knows, from experience, that most inquiries using that phrasing don’t come from serious buyers. They come from informal brokers who learned the fuel’s name from a website, not from a refinery operations manual.

This reaction might seem unfair. In some cases, it is. But it’s happening hundreds of times a day across the industry, and if you’re trying to buy or sell what the informal market calls “D6,” understanding why that reaction exists could be the difference between getting a response from a major supplier and getting forwarded to the spam folder.

Here is the record, set straight: what this fuel actually is, what it’s properly called, and why the terminology gap matters far more than most people realize.

What D6 Actually Is

The fuel informally known as “D6” is not a mystery product. It’s very real, widely used, and important to the global energy economy. What it isn’t is named “D6” by any major standards body, any refinery, any government energy agency, or any serious commodity trading desk.

The correct technical designation, under the American Society for Testing and Materials standard ASTM D396 (the benchmark specification for fuel oils across the United States and widely referenced internationally), is simply No. 6 Fuel Oil. The U.S. Energy Information Administration (EIA) calls it Residual Fuel Oil (RFO), defined as the heavier fraction remaining after the lighter cuts of crude (gasoline, naphtha, kerosene, diesel) have been distilled off.

In marine and bunkering contexts, it’s known as Heavy Fuel Oil (HFO), classified under ISO 8217 into grades such as IFO 380, IFO 180, RMG 380, and RMK 700, where the number refers to the fuel’s maximum viscosity in centistokes. In shipping, these ISO designations aren’t optional shorthand. They’re contractual specifications. A bunker supply contract that just says “D6” would be commercially meaningless to a bunker operator or port terminal.

It goes by other names too, depending on region and application. The U.S. Navy calls it Bunker C. Some older specifications call it PS-400 (Pacific Specification). In power generation contexts, it appears as Heavy Fuel Oil or High Sulfur Fuel Oil (HSFO). In some informal trading circles, particularly in West Africa, South Asia, and online B2B platforms, it picked up the label “D6” or “Virgin Fuel Oil.”

That last term, “Virgin Fuel Oil,” carries its own complications. It has no ISO or ASTM definition. It generally implies unblended residual fuel oil that has come straight from the vacuum distillation column without additional blending or processing. But because it lacks a precise specification, a supplier claiming to offer “Virgin Fuel Oil” and a buyer expecting a specific viscosity, sulfur content, and density profile may be describing two completely different products. Specification disputes before a cargo even loads are the predictable result.

Sources: ASTM D396, ISO 8217, EIA Glossary, U.S. Navy specifications

Where Did D6 Come From?

No one has ever found a single document or a single standards body where the name “D6” was formally assigned to residual fuel oil. It’s a piece of industry folklore that got baked into informal trading networks and spread from there.

The most widely accepted theory is that “D6” is garbled shorthand from ASTM D396, Grade No. 6. At some point, someone abbreviated “ASTM D396 No. 6” to “D6,” the shorthand caught on in broker-to-broker communications, and from there it spread through online trading directories, commodity B2B platforms, and informal market networks, particularly in emerging market trading corridors.

The problem is that the “D” prefix in ASTM nomenclature refers to the designation of the specification document, not the product. ASTM D396 is the document. No. 6 is the fuel grade. Calling the fuel “D6” is roughly like calling EN590 diesel “EN5”: it takes one part of a standards reference and discards the rest in a way that creates confusion.

A secondary theory points to confusion with diesel fuel, where “D1” and “D2” are sometimes used informally to describe No. 1 and No. 2 distillate fuels (also covered under ASTM D396 and ASTM D975). Following that informal sequence, some brokers extended the logic to “D6” for the heavy end of the barrel. The problem is that “D2” itself is informal. Proper diesel is “No. 2 Fuel Oil” or “EN590 10ppm” depending on the specification, so the entire numeric sequence is informal, and stacking “D6” on top of it compounds the confusion.

Whatever the origin, “D6” entered the lexicon and stayed. By the time online commodity trading platforms made it easy to post product solicitations to thousands of buyers and sellers simultaneously, the term had embedded itself deeply enough in certain trading circles that new entrants simply learned it as the product name, often without ever seeing the actual ASTM or ISO specification.

Why Refineries Reject D6 Inquiries

This is the part that new buyers and sellers in the space sometimes find harsh. But it’s worth being direct about, because understanding the reaction saves time and protects credibility.

When a refinery, a major trading house, a terminal operator, or a bunker supplier sees “D6” in an inquiry, the immediate association is not “buyer interested in residual fuel oil.” The association is:

• A broker who is several intermediaries away from any actual product
• An inquiry copy-pasted from a template found on a commodity platform
• A high likelihood of SBLC, MT103, MT760, or “performance bond” language appearing somewhere in the next message
• A low likelihood of a purchase order ever being signed

That’s not snobbery. It’s a pattern that developed because the “D6” solicitation ecosystem became heavily associated with what the industry calls “teaser trade” fraud: elaborate multi-step transaction structures that promise large volumes of product at below-market prices, require the buyer to provide financial instruments upfront, and consistently fail to produce actual cargo. The common language across most of these schemes is “D6 Virgin Fuel Oil” with “FOB Rotterdam” or “CIF any safe port” pricing at 20 to 30% below spot, with an SBLC or MT103 conditional payment structure.

None of that is the buyer’s fault if they genuinely want to purchase residual fuel oil and simply learned the informal name. But the association is real, it’s industry-wide, and using “D6” in first contact with a serious counterpart costs credibility that is very hard to earn back.

The Right Terminology by Context

The right terminology depends on what the product is being used for and who you’re talking to. Here’s a working guide.

Buying for industrial power generation or large boilers

Use No. 6 Fuel Oil or Residual Fuel Oil (RFO). These are the terms your supplier, your logistics provider, and your storage terminal will recognize without hesitation. In a U.S. regulatory context, these terms also align directly with EIA reporting, EPA permit language, and state environmental regulations.

Buying for a marine vessel or fleet

Use the ISO 8217 grade designation, specifically IFO 380 (or IFO 180 for lower viscosity applications), or since IMO 2020, VLSFO (Very Low Sulfur Fuel Oil, maximum 0.5% sulfur) if you need to comply with the MARPOL sulfur cap. If your vessel has an approved scrubber system, you may still use high sulfur IFO 380. Just specify it as HSFO IFO 380. Your bunker supplier will know exactly what you mean.

Trading or brokerage context

The commodity trading and price reporting world uses HSFO (High Sulfur Fuel Oil), HFO (Heavy Fuel Oil), or simply “Fuel Oil” with a viscosity and sulfur specification attached. Platts and Argus quote prices as HSFO 380cst, VLSFO 0.5%, LSFO, and so on. Use these terms in a trading context and you’re speaking the right language.

Government or regulatory context

The U.S. EIA and equivalent government bodies use “Residual Fuel Oil” as the catch-all, with No. 5 and No. 6 as the ASTM-specified sub-grades. Federal Specification VV-F-815C also governs certain procurement. For international regulatory purposes, MARPOL Annex VI governs sulfur content for marine fuels regardless of what you call the product.

No. 6 Fuel Oil: The Technical Facts

For buyers and sellers who need the actual product data, here is what No. 6 Fuel Oil (RFO/HSFO) is as a physical commodity.

It is the heaviest commercial fraction recovered from crude oil distillation. Literally what remains in the vacuum distillation column after gasoline, naphtha, kerosene, jet fuel, and diesel have been drawn off. That’s why “residual” is such a precise and accurate name: it is the residue. The only crude fractions denser than residual fuel oil are carbon black feedstock and bituminous asphalt used for road paving.

Its molecular makeup (long-chain alkanes, cycloalkanes, and aromatics) gives it very high viscosity. At ambient temperature it can approach the consistency of molasses or cold tar. It requires preheating to between 104°C and 127°C (220°F to 260°F) before it flows well enough to be pumped, atomized in a burner, or burned in an engine. This is why residual fuel oil is limited to large installations: power plants, refineries, cement plants, and large oceangoing vessels that have the infrastructure to heat it. You cannot put it in a truck or a small vessel. It will solidify.

Its natural sulfur content is high, typically 2 to 4% by weight in high-sulfur grades, because most of a crude oil’s sulfur concentrates in the heavy fractions. The IMO 2020 regulation under MARPOL Annex VI capped sulfur in marine fuels at 0.5% globally (0.1% in Emission Control Areas), which effectively required the marine industry to switch from standard IFO 380 (HSFO) to VLSFO or to install scrubbers. That regulatory shift significantly altered how residual fuel oil is traded, priced, and specified in the marine market.

Sources: ASTM D396, ISO 8217, MARPOL Annex VI, EIA

Why Terminology Wins Deals

There’s a practical point underneath all of this that goes beyond terminology etiquette.

The global residual fuel oil market moves roughly 3 to 4 million barrels per day, primarily for marine bunkering and power generation in markets without access to natural gas infrastructure. It’s a real commodity with real price indices, real logistics infrastructure, and real buyers and sellers transacting at scale.

The problem is that the informal market, the one that communicates in “D6,” has effectively polluted the inquiry environment for legitimate buyers and sellers. A company in East Africa that genuinely needs 50,000 tonnes of heavy fuel oil for a power plant, and whose procurement team learned the product name as “D6” because that’s what circulates in their market, is sending inquiries that a qualified bulk fuel supplier or refinery will screen out on the first line.

Correcting the terminology takes about five minutes. The credibility it restores can open doors to refinery-direct supply chains that informal broker networks simply cannot reach, where pricing is based on actual market benchmarks, product specifications are contractually guaranteed, and cargoes actually get loaded.

Fuel supply and trading professionals who work at the refinery-access level consistently say the same thing: the single fastest way to signal that you’re a serious buyer is to use the product’s correct name. Not because terminology is a game, but because it signals whether you understand what you’re buying, which in turn signals whether the transaction is worth committing resources to.

The same is true for sellers and brokers. If you’re representing actual supply, using proper nomenclature in your product presentations isn’t pedantry. It’s positioning. It puts your offer in the category of professional supply documentation, not online commodity postings that professionals filter out before their second coffee.

Frequently Asked Questions

What is the correct technical name for the fuel commonly called “D6”?

The correct designation under ASTM D396 is No. 6 Fuel Oil. It is also correctly called Residual Fuel Oil (RFO), the term used by the U.S. Energy Information Administration, or Heavy Fuel Oil (HFO) and High Sulfur Fuel Oil (HSFO) in commodity and marine contexts. In marine bunkering, the ISO 8217 grade designation is IFO 380 or IFO 180 depending on viscosity. None of these names are informal. All appear in recognized international standards.

Where did the name “D6” come from?

“D6” is believed to be informal shorthand derived from ASTM D396 Grade No. 6, the governing specification document combined with the grade number. It is not assigned by any standards body and does not appear in ASTM D396, ISO 8217, MARPOL, the EIA glossary, or any professional commodity price index. It circulates in informal trading networks, online B2B platforms, and broker-to-broker communications, particularly in West African and South Asian trading corridors.

Why do refineries and professional traders dislike the term “D6”?

Because the term has become strongly associated with informal or fraudulent transaction structures, specifically the “teaser trade” pattern in which a broker represents availability of product at below-market prices with financial instrument payment requirements and no actual cargo. Refineries and trading desks receive hundreds of “D6 Virgin Fuel Oil” inquiries for every one that represents a genuine buyer with a real vessel and a real purchase order. The term triggers immediate filtering. Using correct nomenclature (No. 6 Fuel Oil, HSFO, RFO) signals professional intent.

Is “Virgin Fuel Oil” a recognized industry term?

It is used in some informal trading documentation but has no definition in ASTM D396, ISO 8217, or any government energy standard. It generally implies unblended residual fuel oil that has not been cut with cutter stock or blended with other fractions, but because the term lacks a precise specification, it creates ambiguity in product quality discussions. Professional correspondence should specify the product by its grade designation and list key specifications: viscosity, sulfur content, density, and flash point.

What is IFO 380 and how does it relate to No. 6 Fuel Oil?

IFO 380 stands for Intermediate Fuel Oil with a maximum viscosity of 380 centistokes at 50°C, as classified under ISO 8217. It is the most common residual marine fuel and is the marine market’s equivalent of No. 6 Fuel Oil (ASTM D396), with ISO 8217 providing the specific viscosity, sulfur, and density requirements for marine use. The underlying product is very similar. Both are residual fractions from crude distillation. But the ISO 8217 specification adds marine-specific requirements such as stability, compatibility, and ignition quality indices.

How do I buy residual fuel oil (HFO/HSFO) from a legitimate supplier?

Work with a Verified Fuel Supplier that can provide product from a named refinery or terminal with quality certificates, SGS or Bureau Veritas inspection, and a standard commercial payment structure (LC at sight or TT against documents). Specify the product correctly: HSFO IFO 380 to ISO 8217:2017 or No. 6 Fuel Oil to ASTM D396. State your discharge port, volume, and delivery window. Petrolodex connects bulk buyers in Africa, South America, and Asia-Pacific with refinery-access supply for HFO, HSFO, MGO, EN590 diesel, Jet A-1, LPG, and crude oil. Contact us at petrolodex.com.

Know the Product. Use the Right Name. Get Better Deals.

The fuel this industry calls “D6” has been moving in ships and powering generators for over a century under a different name. Residual Fuel Oil. No. 6 Fuel Oil. IFO 380. HSFO. These names appear on the bills of lading, the quality certificates, the port clearance documents, and the delivery receipts of every legitimate cargo ever traded.

If you’re a buyer: knowing the right name opens the right doors. If you’re a seller: using the right name puts your offer in front of the buyers who can actually close.

Petrolodex provides Fuel Supply Solutions for buyers and sellers across Africa, South America, and Asia-Pacific. As a verified global bulk fuel supplier with access to residual fuel oil, HSFO, EN590 bulk diesel fuel, Jet A-1, LPG, MGO, and crude oil, we work with buyers who know what they need and use the language that gets deals done.

In procurement in oil and gas, the difference between an ignored inquiry and a signed contract is often smaller than you’d think. Sometimes it starts with what you call the product.