
I had posted in "interesting articles" forum about an article in this month's Machinery Lubrication which stated minimum recommended viscosity of journal bearings at operating temperature is 13 cSt.
Let me explain, no there's to much to explain let me sum up, journal bearings are what's in every car engine which the cranshaft rides on, and which caries the brunt of the force from the piston pushing down during the power stroke. So for this argument, these journal bearings are the most important in an engine and common sense tells me would be the most significant source of wear in an engine. We want to minimize wear, therefore we want an oil with minimum viscosity such that the shaft in the journal bearing doesn't shear or break through the oil film.
I think there's more to it than this, so please read journal bearings min recommeded viscosity 13 cSt
to hopefully get the article and get the info for where I'm coming from.
Let's take a conservative approach and say that car engines operate over all conditions, low to high speed. So at low speed < 2000 rpm a high viscosity is desireable to prevent metal-metal contact of rotating parts. And at high speeds > 4000 rpm a low viscosity (shear-stable) oil is desireable becuase of hydrodynamic lubrication and fluid (oil) friction/drag. So what oil do you use?
Someone posted a link to a shell viscosity blend calculator. I used it to create chart above. Debate is open to wether the shell calc is accurate and means anything. I plotted the oils you see listed.
I've read in previous machinery lubrication articles which hinted at optimum oil temp in an engine, although for a natural gas industrial type engine, was 190F. Higher than that resulted in oxidation, and higher temps means higher oxidation obviously. Lower than 190F resulted in nitration, and lower than 160F I think they said was really bad for nitration. This too is up for debate. Is it correct, or at least close? What is optimum oil temp?
What I find interesting is for 5w30, the most recommended oil viscosity, that optimum oil temp of 190F it shows 13 cSt. Coincidence? Conspiracy theory? If I disappear that means CAFE and the EPA got me.
I have an oil temp gauge on my car. With an OEM thermostat of 190 or 195F my oil temps run 190-250F. Coolant temps are almost always < 226F, around 200-220F best I can tell. My experience has been that with 30 weight oils in summer the oil temp has been 220-260+ on average depending on driving conditions. Stop+go traffic and no air through radiator = hot aluminum motor @ > 220+ F coolant temps. Radiator fans come on low speed at 226F and high speed at 234F per my service manual.
Running mobil 5w40 truck/suv currently. Observed oil temps have been 200-240F on average depending on driving conditions. On average, it's been lower than a 30 weight oil, and I seem to notice that oil temp cools quicker now with the 5w40: from 240-250 to 220-230 on average. With the 30 weight it would always stay 230+ always until I shut car off for good amount of time. Also, the 5w40 does not heat up quicker compared to a 30 weight, contrary to what the current belief seems to be.
Back to my chart, I referred to optimum oil temp being 190F, great for 5w-30.
However, my engine and maybe most engines run oil temps at 220+ since thermostats are usually 190F. Can we agree that thermostats are 190F for the reason of lower emissions and greater efficiency?
Then, if oil temps are around 220F normally and min recommended viscosity for journal bearings according to what I think is a reputable source is 13 cSt, then aren't 40 weight oils a good thing? And I mean good thing not as better than 30 weight but as in won't do harm like some people claim since the almighty owner's manual doesn't "recommend" a 40 weight oil. I won't ask about a 50 weight oil, yet.
So here's my thoughts, I've shown hard data, I've said where I've got the number's from and asked if we have consensus on what I think are common understandings. Take my personal experience however. Pick it apart, what do you think? At the least make a good discussion.
> my viscosity #'s for 40C and 100C were taken out of the sticky link at top of this forum, oil technical data. These numbers I got were from the manuf's website and you should be able to verify these as well. These were put into the shell calculator to come up with cSt numbers > 212F (100C).