The board says it was actually as flammable as gasoline.
The board took oil samples from nine tank cars that were intact after the crash and subjected them to a rigorous analysis.
The report says the oil from the cars was a Class 3, PG II product, although it had been documented as a less volatile, Class 3 PG III.
The samples were consistent a light, sweet crude oil, with volatility comparable to that of a condensate or gasoline product.
The July 6 crash and explosion killed 47 people and destroyed much of Lac-Megantic's downtown area after a runaway train belonging to the Montreal, Maine & Atlantic Railway rolled into town and derailed.
The board says the lower flashpoint of the oil on the train explains in part why it ignited so quickly once the tank cars were breached.
The downtown was engulfed in a fireball as thousands of litres of oil exploded.
The board's investigation continues.