Oh; never mind the above; it's already being done:
Transformation of Cerium Oxide Nanoparticles from a Diesel Fuel Additive during Combustion in a Diesel Engine
https://pubs.acs.org/doi/10.1021/acs.est.6b03173
What are the Limitations of Using Cerium Oxide in Fuels?
https://www.azonano.com/article.aspx?ArticleID=5374
"...The study scientists noted that by adding cerium oxide nanoparticles to diesel fuel, boosts fuel efficiency,.."
NB that
there is already a commercially available additive called Envirox
https://www.ncbi.nlm.nih.gov/pmc/articles/PMC4066369/
"...we confirmed that the addition of Envirox additive to an ultralow sulfur diesel fuel resulted in a reduction of fuel consumption per unit electricity generated (Figure 1). The fuel savings were at the lower end of the 4–10% savings range...
increasing Envirox concentrations 10-fold (the 10× fuel) caused further reductions in fuel consumption and in emissions..."
Sigh... It'd be nice to be the 1st to think of something for a change!
But NB the negative health effects.