Mundy,
Good point, but add 3 drops of diesel to the additive - what happens? Add 3 drops of the additive to the diesel - what happens? If they both form a uniform solution - they are both mutually soluble in each other. This is the common situation with liquids that are soluble in each other, especially when they are miscible at all concentrations.
Solute and solvent are more appropriately used when liquid solvents are being discussed with solid solutes. Salt is soluble in water, but not in oil. Styrofoam is soluble in acetone but not in water. It makes much less sense to discuss ethanol as a solvent with methanol as the solute. They are both miscible in each other.
tsterkle - Sorry for the OT posts - have you gotten any more information on the engine damage?
jb