I used Forte recently in the Volvo petrol tank and it made no difference to my emissions reading, might be great on another car though.
Engine flush, that is another matter entirely, lots of reading on tinternet about it, some good some bad, Could dislodge sludge to the detriment of the oilways and ultimately the pump ? I have also read that super duper mineral oils can find any weak gaskets or seals and then leak profusely from them ?
I think simple is best with a Jag engine, regular oil and filter changes and drain the oil when it is stinking hot is my favoured approach, good quality 10/40 semi synthetic being my tool of choice.
One thing I would never ever do is put diesel in my engine though, I would hate to think of the damage it would do. I hate diesels with a vengeance so no DERV is getting within 50 yards of 117