Stephen Howard wrote:

Bearing in mind that such tanks are gravity powered and bottom ( or
thereabouts ) draining, whatever was added would need to be light
enough to remain in suspension or at least be whipped up by light
turbulence as the diesel drained out.
All I could come up with was fine carborundum powder.

Any suggestions?

Sugar is the traditional additive for such instances, isn't it? or is
that just for petrol?

David