I have only heard of diesel oil being used by mechanics as a means of removing sludge... Now, in the fuel tank, I wouldn't think this is in the best interest of your fuel pump and injectors.
Cummins Centinel Advanced Engine Oil Management System which is available on L10 and up OTR truck engines takes a small amount of oil out of the lubrication system when the engine is running and pumps it into the fuel tank to be burned in the engine. This oil is replaced in the engine with fresh oil out of an onboard tank. Oil change intervals can be extended to 525,000 miles with this system. "The result is a better protected, better running engine". Quote from Cummins web site.
But this example is oil in deisel fuel, not petrol..
I have also heard of guys using 2 stroke oil in the diesel fuel tank to help lubricate and protect. Again, I would not suspect it is good for Direct Injection type systems.
You best use fuel and oil as per manufacturers recommendations.. unless you want a curious venture to end up as a costly lesson.