touge_ae101 wrote:thin oil can be bad for bearings i used the same oil with and without cooler- all it means is the temperature is stable.
even 0W20 is in same ballpark as 10W40, when it comes to viscosity.
The viscosity is measured in cST.
0W20 has 100'C cST ~ 9.
10W40 has 100'C cST ~ 15.
If you think the almost two fold difference is bad, look what happened when your engine is overcooled.
To put in perspective (which would answer my question from ~5 years ago, that no one answered), the 40'C cST (when the engine is warming up, or overcooled) is following:
0W20 40'C cST ~ 45. that is five fold increase.
10W40 40'C cST ~ 96. that is over six fold increase.
Keep in mind that engines are designed to run cST of around 10 (give a take a few), and definitely not around 100.
So the answer is: No, running too thin oil (providing it is an engine oil) most likely will not run your bearings.
What runs your bearings is oil starvation, if oil is too thick it for oil pump to pump at high RPM. Of course if oil is overheated it will burn and decompose which can also cause bearing failure, hence the need of an oil cooler.
The thinner the oil the easier for pump to circulate. The viscosity does not correlate with lubrication (ie the thicker oil does not lubricate better).
Additionally increasing temperature will not drop viscosity by much. It is not linear correlation (1/x is close curve that can represent it, if the viscosity is on y scale and temperature is on x scale).
For example same 0W20 at 120'C will have viscosity of ~6 cST. at 130'C it will be ~5 cST, and at 140'C it will be just under 5 (4.6).
While the 10W40 @ 120'C ~ 10, @ 130'C ~ 8, @ 140'C ~7.
keep in mind that 10W40 will most likely start breaking down earlier in temperature range than 0W20 because 10W40 will be of mineral base (maybe with some synthetic additive), while 0W20 most likely will be of ester base, which generally is more stable at higher temperatures.