Ugh, not another used motor oil thread.

One of the reasons motor oil is changed is because of it going acidic ( it comes back as a low TBN Total Base Number in a oil analysis. ) . Some put all sorts of filters to protect the injection pump but will burn used up motor oil thinking they are doing good.

Also, motor oil isn't designed to be burnt, it has all sorts of additives that turn to ash / carbon. Do you really want to load the combustion chambers with carbon / abrasive ash? 2 stroke oils make a big deal about being ashless, I'd think they are on to something.

Viscosity improvers in motor oils are polymers, ( plastic ) Do you really want to have such chemicals running through your combustion chambers?

Yes a diesel ( or even a gas motor at reduced percentages ) can run on old motor oil, but in the long term what are you risking / saving in the long term?