Lubrication of the injector pump. The VP-44 injector pump in the second gens are cooled and lubed by the fuel. The new ULSD has had a lot of the lubricating properties stripped out of the diesel due to the desulfurization, therefore its a good ideam to add some extra lubrication to help prolong the life of the pump.

Is there a recommended amount and I assume it is done at fill up

Yes, adding it at fill up ensures that it gets mixed well.

You should add at least a litre per tank.

thanks 4 info