According to Royal Purples tech line they achieve their film strength by ionic bonding.
https://www.rdmag.com/award-winners/2014/08/low-friction-engine-oil
Don’t know if the link provided in Comnoz’comment above refers to the specific ionically bonded anti-wear technology employed by Royal Purple – maybe, maybe not? Even if the article does not refer to the specific Royal Purple technology, it is nevertheless interesting technology that employs ionic liquid oil additives that provide a synergistic effect with oils containing ZDDP. Since the forum readership is now expert in their knowledge of ZDDP chemistry (don’t be bashful you are all experts now aren’t you?), thought you may want to move your understanding of this “ionic liquids” technology beyond the words and see what is actually going on in the oil to provide the enhanced anti-wear performance.
The articles I’ve found on the subject are only abstracts, as permission or membership is required to read the full articles, yet reveal what occurs that provides the synergistic anti-wear effect. The image below is from an abstract by the Oak Ridge National Laboratory group that shows the chemistry involved. I apologize for the quality of the image but it is what it is.
https://onlinelibrary.wiley.com/doi/abs/10.1002/adma.201502037
To understand the image let’s begin by identifying the chemicals present. On the upper left of the image you’ll recognize your old friend ZDDP (the Zn dithio blah, blah, blah, oriented vertically in this case) with a dashed orange box around the lower half of the ZDDP molecule. Then just to the right of ZDDP (past the “plus” sign) are the ionic liquids, consisting of a phosphonium cation shown on top and having a positive charge (has the 4 R groups oriented about the P atom), and below it encircled with a solid green line is the anion having a negative charge. Moving further right past the reaction arrow are the new products formed. The molecule shown in solid green is the one to keep your eye on as this is the new molecule with synergistic anti-wear properties. Examining the structure of the new molecule you’ll note that the top half of the molecule is unchanged ZDDP, but the bottom of the ZDDP molecule has been replaced by the ionic liquid anion.
I believe (but don’t know) that the legend/key to the performance graphs is as follows. ZDDP is shown in blue, and the phosphonium ionic liquids are shown in orange, where both additives, employed as the sole additive, provide reasonable anti-wear performance on their own. However, the novel performance lies in the synergy that occurs when the orange and blue are combined to provide the new green molecule.
The last link below names a specific phosphonium ionic liquid that when combined in a fully formulated motor oil provides the above described synergy in anti-wear performance. Some of the same authors as above, but earlier publication date.
https://pubs.acs.org/doi/full/10.1021/am201646k
As mentioned previously, the formulated cost/performance ratio of any oil we buy is unknown, but I wouldn’t be the least bit surprised if additives like those described above might add significantly to the raw material cost of a motor oil, hence in some cases the price may relate to performance and "what's in the bottle".