Stribeck Curve and Oil Selection

First of all, what is the Stribeck curve?
Simply put, it is a simplified graph of the relationship between oil film thickness and resistance to load.

Boundary - Mixing - Liquid

vertical axis: friction coefficient
Horizontal axis: Viscosity*speed/load

Let's look at the graph from the left.

First, the area where the line is high _ i.e., the high resistance position is followed by a flat area.
This is called "boundary lubrication. There is not enough oil film at all,
The film or bonded molecules on the metal surface or surfaces are rubbing against each other.

Past this point, there is an area that goes right down to the bottom.
This is called "mixed lubrication," where parts rub or do not rub against each other.

And beyond the lower peak is "fluid lubrication."
This is the state where there is enough oil film and the parts are completely floating.

Looking at the finer details, though, there is something called "elastohydrodynamic lubrication" between fluid lubrication and boundary lubrication,
I will not go into that much detail this time, so I will skip it.


Looking at this graph, you can see where we are aiming for the lowest resistance.
Yes, this is where the oil film is thinnest in fluid lubrication.

However, this is where the problem arises.
The engine is not evenly lubricated.
Since oil is pumped against gravity, the oil film is inevitably thinner at the top.

Then, for example, if the crankshaft in the low position is brought to the best position,
The oil film on the pistons and cams in the higher position will be insufficient, resulting in increased resistance and wear.
On the other hand, if you try to lubricate everything fluidly with an emphasis on protecting the pistons and cams,
crankshaft area, the oil film will be too thick and the resistance will increase.
This is a difficult problem.

In recent years, cars have adopted roller rockers and DLC coating to reduce friction and wear at the weak point,
Even if low viscosity oil is used, it is designed so that resistance will not increase or abnormal wear will not occur.
However, in older cars that use 5W-30 or higher as the specified oil, measures that are not as good as those used in recent years have not been taken.
Therefore, in order to prevent cam and piston wear and reduce overall engine resistance, a harder oil is selected.