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Premium model or not, you can't change the laws of Physics (or chemistry). Rubber degrades over time - and you are already aware of some of the affects.
The tires are 8 years old, and I have heard anywhere from 6 to 10 years as a tire's life depending on where you live - hot climates (AZ, TX, CA, NV and FL): 6 years. Cold Climate (MN, ND, ID, MT, WI): 10 years
This is regardless of whether the tires are worn or not.
So I would seriously consider replacing them just due to the age. Tire failures can have tragic results and you just do not want to be part of that statistic.
But to answer your question: Diesel fuel is not a good idea. Rubber absorbs petroleum products and dissolves the rubber bonds - weakening the tire.
The only thing I am aware of that might be useful is the tire treatments that racers use - Formula V comes to mind. I have used this on my old race tires and it works. However, I have no idea if what it does is appropriate for street tires.
I've read some discussions about what is in these products and I think the possible suspects are all solvents of some sort, flammable, and carcinogens.
Overall, this doesn't sound like a good idea.
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