Heavy fuel oil: what "380" does not tell you about a cargo

A heavy fuel oil offer often comes down to three characters: "380". That number describes a viscosity, and nothing else. It says nothing about sulphur, nothing about density, nothing about the impurities that will decide the service life of your engines or your boiler. Two products sold under the same "380" can be separated by a technical gulf and by a considerable price gap.

Heavy fuel oil is the only petroleum product that buyers specify by its thickness before they specify its composition.

What "380" actually designates

It is the maximum kinematic viscosity at 50 °C, in mm²/s, a unit still commonly called centistokes. A 380 fuel oil is therefore a product whose viscosity does not exceed 380 mm²/s at that reference temperature. The other common grades, 180 or 500, are read the same way.

Viscosity governs all handling: pumping, filtration, preheating, injection. It says nothing about the quality of the product. It is a logistics parameter, not a compliance one.

ISO 8217 defines grades, not a single product

The international reference standard is ISO 8217, whose current edition dates from 2024, following those of 2017 and 2012. It classifies residual fuels in grades designated RMA, RMB, RMD, RME, RMG and RMK, followed by the viscosity.

Two grades carry the same 380 viscosity and yet are rarely interchangeable in practice:

• RMG 380: maximum density of 991.0 kg/m³ at 15 °C.
• RMK 380: maximum density of 1,010.0 kg/m³ at 15 °C.

The difference is not cosmetic. Above 991 kg/m³, separating water by centrifuge becomes impossible with conventional separators, which rely on the density difference between water and fuel. Accepting an RMK on an installation designed for RMG means giving up on removing water from the product.

The other RMG 380 limits set out the rest of the requirement:

• CCAI: 870 maximum. This index, calculated from density and viscosity, estimates the ability to self-ignite. A high CCAI signals ignition difficulties.
• Flash point: 60 °C minimum. A lower value signals contamination by a light product.
• Water: 0.50 % by volume maximum.
• Ash: 0.100 % by mass maximum.
• Total sediment after ageing: 0.10 % by mass maximum.
• Vanadium: 350 mg/kg, sodium: 100 mg/kg maximum. Together they cause hot corrosion of valves and turbines.

The standard does not set the sulphur limit

Here is the point most buyers get wrong. In the ISO 8217 tables, the "sulphur" line carries no value: it refers to the applicable regulation.

In other words, the standard does not define the permitted sulphur, the regulator does. An RMG 380 perfectly compliant with ISO 8217 may contain 3.5 % sulphur or 0.5 %: both comply with the standard, only one may comply with your use.

For ships, MARPOL Annex VI decides:

• 0.50 % by mass maximum worldwide, since 1 January 2020, against 3.50 % previously.
• 0.10 % in emission control areas, known as ECAs: Baltic Sea, North Sea, North American and United States Caribbean areas.
• The entire Mediterranean became such an area, with the 0.10 % limit applying there since 1 May 2025.

A ship fitted with an exhaust gas cleaning system may keep burning a high sulphur product, since the requirement bears on the emission and not on the fuel.

For use on land, none of this applies. A thermal power plant or a cement works does not fall under MARPOL but under the national regulation of the operating country, which may be looser or stricter. This distinction is regularly confused in tenders, resulting in cargoes that are either overpaid for or non-compliant.

Catalyst fines, the parameter that destroys engines

If only one parameter beyond sulphur were to be remembered, it would be this one.

Catalyst fines are aluminium and silicon particles carried over from catalytic cracking at the refinery. They are extremely hard and abrasive. ISO 8217 caps aluminium plus silicon at 60 mg/kg.

The trap fits in one sentence: that limit applies at delivery, not at the engine inlet. Fines concentrate by settling at the bottom of tanks. A product received at 40 mg/kg may show considerably more in the last tons pumped, and purification on board or at the plant must bring the value down to around ten mg/kg before injection.

An engine fed with catalyst fines wears out its liners and rings within a few hundred hours. The damage is slow, silent, and almost never traced back to the supplier by the time it shows.

Stability and compatibility, the issue born in 2020

Before 2020, heavy fuel oil was a fairly homogeneous product from one port to the next. The move to 0.50 % brought in fuels blended from very different base stocks depending on the refinery.

Two products can each be perfectly compliant and become incompatible once mixed: the asphaltenes of one precipitate on contact with the paraffinic components of the other, and the mixture produces sludge that clogs filters and separators. Commingling two parcels of different origin in a tank is therefore a risk operation, not a convenience.

Two simple precautions: do not mix two supplies without a compatibility test, and track total sediment after ageing, which remains the best available indicator of the stability of a parcel.

Heating, pumping, injecting

A 380 fuel oil cannot be pumped at ambient temperature. The whole chain assumes it is kept hot:

• In storage, above the pour point with a safety margin, failing which the product sets in tanks and heating coils.
• Before injection, viscosity has to be brought down to around ten mm²/s, which requires heating a 380 to somewhere between 135 and 150 °C depending on the equipment.

In tropical zones, the pour point question often takes second place. It becomes central again as soon as an installation stops for several days, or a tank without heating tracing is left at the bottom of stock.

Five points to write into the contract

1. The full ISO 8217 grade, RMG 380 or RMK 380, not just "380".
2. The edition of the standard applied, 2024 or 2017, since the limits have changed.
3. The sulphur content as a figure, since the standard does not set it, with the destination regulation as the reference.
4. The aluminium plus silicon limit, and the sampling point that will govern.
5. Total sediment after ageing, the only contractual safeguard against an unstable parcel.

Key takeaways

• "380" is a viscosity, not a quality.
• RMG and RMK are not equivalent: above 991 kg/m³, water no longer separates.
• ISO 8217 does not set sulphur, it defers to the applicable regulation.
• On land, MARPOL does not apply: national law decides.
• Catalyst fines concentrate in tanks and destroy engines quietly.
• Two compliant products can be incompatible with each other.

Sulphur content follows the same logic as for diesel: it is a matter of destination market, a subject covered in our article on sulphur limits by market.

Our heavy fuel oil specifications are confirmed cargo by cargo, with the grade and the edition of the standard stated. For a specific requirement, request a quotation.