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Blending diesel for winter conditions can impact the energy density. "Higher density fuels generally have higher energy content, meaning better fuel economy. But generally, these fuels are not viable for cold climates," says Gault. No. 1 fuel or fuel blends of No. 1 and No. 2 tend to be less dense, and therefore lower in energy content. "But they are required to prevent fuel flow problems in cold conditions."

Ignition quality
Cetane number is an indication of how easy the fuel is ignited in your engine. The higher the cetane number, the shorter the ignition delay between the start of injection and combustion of the fuel. "The higher the cetane number the better, if all other aspects of the fuel are equal," says Gault.

Currently, No. 2 diesel fuel has cetane values that range from 40 to 55. "ASTM D-975 specifies a minimum cetane number of 40 for any diesel fuel," says Harvey. "Most owner manuals also require a minimum cetane number of 40." For winter fuel, you should look for a cetane level closer to 50.

"As the cetane number increases, you can expect better cold temperature starting, improved combustion and emissions, in addition to a quieter running engine," says Theisen. "We know that premium-type diesel fuels tend to have higher cetane numbers."

If the cetane number is too low, the engine will be more difficult to start, particularly in cold weather, and it may run rough. Oil sludge may accumulate more quickly, along with engine deposits. Yet, there is no benefit to running a cetane level higher than engine manufacturer specifications.