Julian Macqueen has written extensively on the shipping industry.
The product you buy is the same as the one you think you are going to buy.
It's a straightforward enough proposition.
Yet, in the bunker market, you can never be sure that what you pay for is what you get, which is something of a worry to Iain White, global field marketing manager with ExxonMobil Marine Fuels and Lubricants.
Bunker fuel is made up of what's left from a barrel of crude once the higher value stuff has been lifted out.
And these days, more of the higher value molecules are removed, leaving behind material with a greater propensity for instability when blended.
Hence, the bunker fuel available to the market is, according to White, becoming "more challenged".
"Refineries are upgrading their plant which allows them to extract more from a barrel of crude," he explains.
Also contributing to fuel's potential instability is the increase in blending to meet the demand for the low sulphur material required by ships operating in emission control areas.
Then there is the human factor.
The understanding of those buying the fuel is, perhaps, "not as good as it should be", White suggests.
Guidance is available in the form of fuel oil specifications.
The ISO 8217 standard has the older 2005 and 2010 versions, and an updated 2012 one. The latter is the standard adhered to by ExxonMobil.
Catalytic fines, or cat fines, refer to the aluminium and silicon content of marine fuel, an abrasive matter that can cause engine damage.
Under the 2012 standard, the acceptable level of cat fines falls from 80 to 60, but buyers will still purchase fuel going under the 2005 standard.
"Why would you do that when the 2012 standard offers a far better profile?" asks White.
It's a pertinent question.
"The 8217:2012 standard is not a manufacturing specification or a quality standard. It's a commercial specification," he says.
The best way for the buyer to protect himself is by testing.
"If you don't analyse the material, you won't know how to treat it."
In fact, the only guidance you will be left with is what's on the bunker delivery note.
All the more reason to test first, he says.
"Without analysis, you simply don't know what you are buying."
How important is analysis, then?
"It's essential," says White.