What exactly are friction modifiers?

[oilyriser]

I suspect FMs work in the EHD regime. When oil is subject to extreme pressure, viscosity ramps up, and drag increases. FMs probably interfere with whatever molecular interaction creates this increase. This would predict that PAO lubes would require less FM. I'll wait for the experts to comment.

 

[aquariuscsm]

So I`m wondering,if the fm`s are different in HDEO`s,how does their lubricity and protection differ from PCMO`s? Are their pros and cons to using either one in a gasoline engine?

 

[JAG]

Lower coefficient of friction in the boundary regime is a desirable trait in gasoline engines.

There are so many different types of FMs, so it is hard to generalize cons. Some will increase sulfated ash (those containing metal atoms like Mo) while many contain sulfur. Cat. converters don't appreciate either of those, but the engine itself generally does. They increase cost to the formulator which the bean counters don't appreciate.

PCMO vs. HDEO protection in a gasoline engine cannot be generalized based on presence or lack of FMs. Coefficient of friction in the boundary regime probably can be generalized (with expected outlyers and exceptions) between the two classes of oils on an equal viscosity basis: PCMO usually less coeff. of friction than HDEO.

As MolaKule's post in the linked thread says, the term is lubricity is "now considered obsolete".