Originally Posted By: Valvman
It's well known in the industry that many sulfur containing compounds (not just MoDTC) can cause issues with "yellow metal" corrosion. MoDTC or similar compunds have been shown to cause this issue in poorly formulated lubricants, but it is fairly easy to "fix" the formulation with well known additives so the corrosion is not a problem even up to high treat rates of MoDTC (well over 1000ppm of Moly).
I think that is true in most applications. Chemistry is such a complicated phenomena so there are exceptions to the rules we try to create. Temperature is one variable that can be one of those exceptions...diesel engines can have higher CC temperatures than gas engines so usage of MoDTC and some other (soft metal sensitive) moly-free sulfur-containing additives in HDEOs must be particularly scrutinized.
From R.T. Vanderbilt Company, Inc.
Quote:
MOLYVAN 807
MOLYVAN 807 offers a unique molybdenum-sulfur combination in an oil-soluble form which is easy to blend into lubricants. MOLYVAN 807 can be used to maintain the antifriction properties of an engine oil while reducing the phosphorus content. To obtain significant increases in extreme pressure properties and impart improved antiwear performance, MOLYVAN 807 can be used in combination with VANLUBE® 7723, a non-metallic dithiocarbamate which functions as antioxidant and extreme pressure agent.
Note: Since oil formulations vary it is recommended that upon the incorporation of any additive the finished product be tested to confirm original test results. In some formulations, the presence of MOLYVAN 807 may contribute to copper corrosion which would be detrimental to some diesel engines. Therefore, its use in such lubricants is not recommended.
Quote:
MOLYVAN 822
MOLYVAN 822 may be used to maintain or improve the antifric-tion properties of an engine oil while reducing the phosphorus content. MOLYVAN 822 is less corrosive to copper than most other organic molybdenum compounds. MOLYVAN 822 is not recommended for use in diesel engine lubricants.
http://www.rtvanderbilt.com/ILSAC GF-5 Development.pdf
Quote:
Fuel Economy and Friction Modifiers A formulator may choose to increase the friction modifier level in order to achieve the required fuel economy increase in the Sequence VID engine test. Many friction modifiers are antagonistic towards Cu/Pb bearing corrosion in the Sequence VIII engine test. Certain organic, vegetable oil-derived friction modifiers are aggressive towards lead34. This effect will be exacerbated in GF-5 if ZDDP is lowered from 0.075% to 0.065%, as ZDDP is a good metal protectant and helps to protect Cu/Pb bearings from corrosion. In this case, other additives can be used to passivate the Cu/Pb surface. Certain molybdenum friction modifiers can also be corrosive but, unlike organic friction modifiers, molybdenum compounds also bring strong oxidation, wear and deposit control benefits to the formulation.
There is more on this subject online that is ready for the picking.
