Cetane is a measure that applies to diesel fuel. It has no relevance to gasoline.
The fuel used in the lawnmower was gasoline.
The relevant measure for gasoline is the Octane number.
If Thecisbest was able to get a gasoline lawnmower to run that well with diesel fuel, then I am truly impressed.
As far as your reference to Viscosity index as having something to do with the Naptha sale, the first line of the link you provided should make it clear that VI has nothing to with Naptha.
"Viscosity index (VI) is an arbitrary measure for the change of viscosity with temperature. It is used to characterize lubricating oil in the automotive industry."
Please note: "It is used to characterize lubricating oil in the automotive industry."
Naptha is a not a lubricating oil use in the automotive industry.
Naptha is used as input to a number of chemical processes, including making high octane gasoline and the manufacture of plastics.
As you may have discovered in you researching Naptha is an imprecise term that covers a range of hydrocarbon mixes. I suspect that is why the 8K had the specification of the boiling temperatures included to be more precise about exatcly what hydrocarbons were being included in the agreement.
OMG! It's an Inverted Spinglefluke formation. Nothing left to do except release the KRAKEN.