Cheers Neale, it was worth the effort to chat, as we were able to between us, confirm some suspicions, corroborate further some evidence of prior knowledge, and generally get some real information as to the vehicles short but turbulent history prior to my ownership and continued development.
We also now know when the additive was added prior to sale and by whom (again, not your shop), and why service records in the log were not completed at your shop, as you did not conduct the service on that occasion, merely diagnosed and identified the defect and its significance to the prior owner, and you opened the door for a warranty claim with the original engine supplier for the previous owner, that, for what ever reason, they chose not to pursue, rather, they took the incorrect and illegal option as detailed in this topic.
For those still unaware of the laws regarding sales of vehicles...
No, a private seller does not have to warrant a car in the same way as a Licenced Motor Dealer.
But, it is a basic principle of the Law of Contract that if a person enters into a contract that is fraudulent, then that contract is voidable.
And if that's the case then, for example, a "seller" can be made to take the object sold
(be that a car or whatever) back, and be made to return the money.
Or, in the alternative, be made to pay for relevant repairs.
Without referring to any speific particular case, an example of a fraudulent act in contract could be
deliberately withholding information about the mechanical condition of a car
that a seller knew, or reasonably should have known, was the kind of information
that a reasonable person would think was relevant to the decision to buy that car or not.
Even if neither party understood the technical/mechanical nature of that information.
Yes, it is a matter for one party (a plaintiff, such as dissatisfied private car buyer)
to show, on the balance of probabilities, that the other party
(a defendant, such as a private car seller) acted fraudulently when entering into the contract.
But that is very do-able - because proving a negative is quite difficult for a defendant to do.
Direct from the a contract and commercial law specialist.