There is a very good long Japanese moly-study paper where they go from 0 up to 1,000 ppm or so moly and try various types and combinations of moly (dinuclear, trinuclear, etc.) and ZDDP (primary, secondary, etc.) but I can't find it. If I find it, I will post it. One conclusion was that trinuclear moly wasn't as good as dinuclear, which I was surprised at. ZDDP type also makes a huge difference. I think primary ZDDP was better.
The Chevron patent about GTL and pressure - viscosity coefficient I posted here specifically says that trinuclear moly shouldn't be used, saying among other things that it has no antioxidant properties, but that could actually be because of an Oronite - Infineum fight over which moly is better.
I dug the moly paper I mentioned and it actually turned out to be two different papers. There is a lot of information to absorb in them.
(1) (
Link) Influence of the alkyl group of zinc dialkyldithiophosphate on the frictional characteristics of molybdenum dialkyldithiocarbamate under sliding conditions
Masayoshi Murakia ^a and Hisayuki Wadab ^b
^a Department of Mechanical Engineering, Shonan Institute of Technology, 1-1-25 Tsujido Nishikaigan, Fujisawa, Kanagawa 251-8511, Japan
^b Lubricants Research Laboratory, Nippon Mitsubishi Oil Corporation, 8 Chidori-cho, Naka-ku, Yokohama 231-0815, Japan
(2) (
Link) Single-cam tribometer for evaluating tribological parameters and tribochemistry of valve train
S. Ashworth, K. Mistry, A. Morina and A. Neville
Institute of Engineering Thermo-fluids, Surfaces and Interfaces, School of Mechanical Engineering, University of Leeds, Leeds, UK
The first article, which you may not have full access, studies three different types of ZDDP (two primary and one secondary) along with dinuclear moly for various moly concentrations. The optimal Mo concentration could be as little as 200 ppm or as high as 700 ppm, depending on the type of ZDDP. Also, different ZDDP types lead to different minimum friction coefficient (and antiwear properties, which may get better or worse as friction gets lowered).
The second article compares PAO, PAO+ZDDP, PAO+ZDDP+MoD, PAO+ZDDP+MoT, and PAO+MoD, with MoD and MoT being dinuclear moly and trinuclear moly, respectively. Dinuclear moly works better than trinuclear. However, while the lowest wear was also found for the PAO+ZDDP+MoD combination for the cam/shim test that simulates a sliding cam, the lowest wear for a pin sliding on a flat plate was found with PAO+ZDDP and addition of moly increased wear in that case.
Therefore, this antiwear/extreme pressure/friction modifier (AW/EP/FM) business is extremely complicated with many different combinations of many different compounds with different concentrations being possible, with no clear optimal material, combination, concentration, or application for a given engine part.