What do we mean by "friction modified"?

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Jun 16, 2003
Franklin, OH
.....and how does this apply to Rotella T Synthetic 5w-40 in a gasoline engine vs Mobil 1 5w-40 and 0w-40? I thought being friction modified was more desirable, yet I see Rotella T Synthetic sometimes recommended over the Mobil products. I'm only interested in running the oil at 5-7K mile intervals.
There are many additives that can decrease friction. I think certain esters can and even ZDP. Moly is a friction modifier. Certain oils that are heavily friction modified can save fuel and reduce wear. My friend replaced Mobil 1 5w-30 and put S2k 0w-30 in his Vette and noticed lowered engine temps and more HP. (Didn't dyno it). I noticed more sluggishness with Delvac 1 over Mobil 1. Could have been the thickeness of the oil though.
The oil I use states categorically "Non Friction Modified". There is some concern (true or false) that FM oils cause uneven wear on roller rockers. Dave
Dave, I take it you are using the fairly expensive stuff in the black bottle, that reckons anything under 20W-60 is too light for Australian conditions. I'd change brands regardless.
It is my understanding that friction modified lubricants is in regard to the internal fluid frictions of the fluid itself and not relating to the lubricants ability to reduce wear and friction on moving parts. You'll notice that most fluids that are friction modified are the lower viscosity oils that wear the 'energy conserving' label. You can buy both a 10w-30 that is friction modified and a 10W-30 that isn't friction modified. This doesn't mean that one oil will reduce your engine friction more. It means that one oil has been modified to require less energy during pumping. My older Volkswagen owners manual specifies oils based on whether they're friction modified or not. Strangely enough, when it mentions using non-friction modified oils the viscosity requirements are increased.
Now I'm confused. Why aren't diesel oils friction modified and why are they recommended so often? It seems that a friction modified oil is better, adding another level of protection to your lubrication before resorting to the anti-wear additives.
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