purpose of Magnesium in oil?

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Staff member
Jun 5, 2002
Iowegia - USA
From this thread: http://theoildrop.server101.com/cgi/ultimatebb.cgi?ubb=get_topic;f=21;t=000032
Detergents (Surface Protective Additive): metallo-organic compounds of sodium, calcium, magnesium, boron phenolates, phosphates and sulfonates such as alkylbenzene sulfonic acids, alkylphenol sulfides, alkylsalacyclic acids; Lift deposits from surfaces to keep them suspended. Dispersants (Surface Protective Additive): Alkylsuccinimides, alkylsuccinic esters (alkenyl succinimides); chemical reaction with sludge and varnish precursors to keep them acid neutralized and to keep them soluble. Detergent-dispersants often are the same chemical or come in compounds to accomplish the combined function(s).
Calcium and magnesium are both white-to-silvery-white metals that are "base" metals. Base metals attack acids and turn them into salts before they can cause corrosion. These Base metal detergents and dispersants are also responsible for establishing the primary TBN of a lubricant. Magnesium is a harder metal than calcium and can cause hard deposits on top rings and crowns of diesel engines. Magnesium additives in too high a concentration can also cause gelling of the oil. "Most" of today's modern oils have lowered the magnesium or dropped it completely in favor of newer non-organic detergents. Much depends on additive interactions as to how much magnesium is used. There are also different Magnesium sulfinate and phenate formulations, and the latest formulations present no problems. Like ZDDP, it still a low-cost cost and tried-and-true compound for most oils.
And what is its advantage over calcium ? If magnesium is used in addition to cacium, then it has to bring something that calcium cannot do. Would somebody clarify ?
is it an anti-wear additive? If not, then what? I was under the impression it is not a "bad" additive, however some people here on bitog have stated that it is undesirable. For what reasons, I don't remember. I think there may be some truth to that, since most passenger car motor oils have little magnesium showing up in UOA and VOA. But in oils like delvac1300, and maybe other diesel oils, there is a decent amount of it, and delvac is a good oil. Another oil that had moderate amounts of Mg was Mercruise 25w-40 marine oil, which I had done a UOA on. So what's the deal?
It is a detergent. There have been some studies done that show high levels of magnesium result in more upper cylinder wear. Shell states flat out this is why they use a calcium based detergent package in their Rotella oils and no magnesium.
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