There seems to be considerable confusion in the recent posts about what octane represents and how that relates to gasoline - diesel blending.
Let's start on the diesel cycle side of things. Diesel cycle is compression ignition. The goal is ignition when the fuel charge is injected and injection timing is critical. Diesel fuel is blended to ignite at relatively low temperatures so it will intentional ignite immediately when injected. Diesel is not evaluated for octane, however if it were it would have an octane rating in the vicinity of 15 - 25 octane …. very low octane with respect to most gasoline.
Spark ignition engines need a different design character in their fuels. While diesel cycle engines WANT and NEED compression ignition, spark ignition engines DO NOT want compression ignition. Straight diesel in a spark ignition engine of even moderate compression will destroy the engine due to Pre-Ignition (compression ignition), that is ignition occurring too early in the compression phase of the rotational cycle due to the heat of compression. Spark ignition engines need fuel which is resistant to compression ignition. That resistance to compression ignition is measured as octane. Higher octane is more resistance to compression ignition.
Diesel fuel has more energy per unit of volume than gasoline.
Modern gasoline ignition timing controls (Computer & sensors) are able to adjust the spark ignition timing in real time, within reasonable limits. They are able to retard the spark ignition timing sufficiently to compensate for small decrements in fuel octane. Blending diesel into gasoline decreases the octane rating of the blend from that of unblended gasoline. If blended rationally it can result in a blended octane result which is within the range of control the computer can exercise and result in complete combustion with no pre-ignition and no damage. The amount of diesel in the blend controls the amount of octane loss. Some vehicles will not tolerate any octane reduction, others tolerate a significant reduction.
Be thoughtful, be rational and experiment carefully starting with very little diesel in the blend if you choose to experiment. It's rather simple combustion physics. If you do it correctly you can reap a benefit. If you do it wrong you will destroy your engine. I'm doing it correctly and all components of my van are working just fine and have been now for seven years.