[lclemente]

Does anyone know how putting diesel into the motor oil causes the engine to "[run] more smoothly and [use] less fuel"?

[kenz0r]

It thins the oil, so there are less frictional losses, at least until the bearing surfaces get damaged.

[chrismbarr]

So why isn't oil like this normally? Are the downsides to doing this?

[jacquesm]

Yes, it shortens engine life.

[maxerickson]

There is a general trend towards specifying lower viscosity oils:

http://www.consumerreports.org/cro/news/2012/09/fuel-economy...