A "Lubricity Additive Study" done when Opti-Lube XPD first came out showed it the most effective lubricity-improver/wear-reducer of 19 additives tested, much better than Power Service brand(on lubricity alone, they both serve other purposes). Since then Opti-Lube came out with this Opti-Lube XL which is supposed to have even higher lubricity than the XPD, possibly the highest available based on my research, though I'm probably not aware of every product out there, nor have the means to test them. However the XL doesn't have the other properties of this XPD(as with PS products), it's just for lubricity, which seems like a good idea for ultra low sulfur diesel. Curious what lubricant makes these so effective? Probably a trade secret. Cheers!