Using thicker oil in an older engine?

Viscosity is determined by 3 factors.

1. Operating oil temp
2. Rod and main bearing clearance
3. The load on those bearings

I disagree. I do understand the gist of your explanation, but it's inaccurate. The first point is salient, the other two are not.

Most simply and accurately described, "viscosity" is the resistance to flow. Viscosity is determined by the chemical composition of the fluid; be it a simple chemical like H2O, or complex ones like fully-formulated automotive lubricants ... and it is generally subject to changing with temperature. You cannot discuss viscosity without stating a temp reference. It's not unlike discussion of HP; that is only relevant to a specific torque at a specific rpm.

However, clearances have no ability to set the viscosity of the fluid, nor do bearing loads. (unless you want to argue that high loads increase flow resistance, but that really just goes back to the reactionary thermal state of "temperature" ... )
 
I acknowledge your disagreement. Ultimately, I'm simplifying things a bit. There's many factors in this such as pressure-viscosity coefficient, dynamic viscosity, etc... plus a buffer. However, this isn't my opinion. In speaking with multiple tribologists and lubricant engineers currently in the field, lubricant company owners, engine builders in professional motorsports, and engineers from Cummins and GM, they all stated the same thing. The bearings decide the viscosity because they're carrying the load. Higher viscosity (dynamic) will carry a higher load and a wider clearance wants a thicker oil film to control crank journal eccentricity. If any other part of the engine wants something different, too bad. The bearings take priority.

Given a typical 220°F sump temp... (generalized)

.0016-.0021" = 20 grade
.0022-.0027" = 30 grade
.0028-.0033" = 40 grade
.0034-.0040" = 50 grade

This is a generalized guide. It's a starting point that will be accurate the vast majority of the time. It obviously will change with temperature which is why I noted temperature first. Specific applications can and do vary from this at times depending on the mountain of variables at play from bearing width and diameter to rpm to block/rod material and so on. Driven came up with their own chart for this as well.

Driven viscosity grade chart.webp
 
40-weight is good for an old 305 engine; they weren’t strong to begin with, so a decently thick oil helps keep them going…
What’s it in???
 
I acknowledge your disagreement. Ultimately, I'm simplifying things a bit. There's many factors in this such as pressure-viscosity coefficient, dynamic viscosity, etc... plus a buffer. However, this isn't my opinion. In speaking with multiple tribologists and lubricant engineers currently in the field, lubricant company owners, engine builders in professional motorsports, and engineers from Cummins and GM, they all stated the same thing. The bearings decide the viscosity because they're carrying the load. Higher viscosity (dynamic) will carry a higher load and a wider clearance wants a thicker oil film to control crank journal eccentricity. If any other part of the engine wants something different, too bad. The bearings take priority.

Given a typical 220°F sump temp... (generalized)

.0016-.0021" = 20 grade
.0022-.0027" = 30 grade
.0028-.0033" = 40 grade
.0034-.0040" = 50 grade

This is a generalized guide. It's a starting point that will be accurate the vast majority of the time. It obviously will change with temperature which is why I noted temperature first. Specific applications can and do vary from this at times depending on the mountain of variables at play from bearing width and diameter to rpm to block/rod material and so on. Driven came up with their own chart for this as well.

View attachment 262997
That marketing piece with nonexistent grades has never meant much that I’ve relied upon. But maybe that’s just me.
 
I acknowledge your disagreement. Ultimately, I'm simplifying things a bit. There's many factors in this such as pressure-viscosity coefficient, dynamic viscosity, etc... plus a buffer. However, this isn't my opinion. In speaking with multiple tribologists and lubricant engineers currently in the field, lubricant company owners, engine builders in professional motorsports, and engineers from Cummins and GM, they all stated the same thing. The bearings decide the viscosity because they're carrying the load. Higher viscosity (dynamic) will carry a higher load and a wider clearance wants a thicker oil film to control crank journal eccentricity. If any other part of the engine wants something different, too bad. The bearings take priority.

Given a typical 220°F sump temp... (generalized)

.0016-.0021" = 20 grade
.0022-.0027" = 30 grade
.0028-.0033" = 40 grade
.0034-.0040" = 50 grade

This is a generalized guide. It's a starting point that will be accurate the vast majority of the time. It obviously will change with temperature which is why I noted temperature first. Specific applications can and do vary from this at times depending on the mountain of variables at play from bearing width and diameter to rpm to block/rod material and so on. Driven came up with their own chart for this as well.

Ah ... I think I see what you're getting at.

Again, I'll nit-pick, but not meant as offense to you, just the way you stated it. If this comes off rude then I certainly apologize in advance; not meant as an affront to you personally.

If I understand you correctly, it would be better stated from your point of view in that bearing clearances are part of the determining factor in the selection of a viscosity grade. It's not that clearances determine the viscosity of a fluid, but to avoid damage, the clearances must be maintained by a lube which can provide a viscosity sufficient to hold the MOFT (often assured by grade or HTHS specs).

When you stated it previously (hence my initial objection), you said "viscosity is determined by 3 factors". I took that as the lubricant viscosity is manipulated by those three things (I do agree that temp directly affects it; the other two, no).

But what you are really trying to convey is that bearing loads and clearances dictate a specific grade for the acceptable minimum film barrier, etc. And that, in turn, is how the OE determines the viscosity recommendation. Your point became more clear when you said "bearings decide the viscosity".

It's not that bearing clearances actually affect the viscosity of the lube. Rather, the bearing clearances need a MOFT barrier, and that is criteria used to select a lubricant of grade "X" to satisfy that requirement. Viscosity of the lube isn't "determined" by those characteristics. Viscosity desired to maintain those clearances results in a selection of the recommended grade (or HTHS value). Bigger clearances dictate the need for thicker vis.

My view is that viscosity of the lube is determined by chemical composition and temperature. However, I can import your points into my view by agreeing with you, in that bearing clearances need a lube with a MOFT, and that is satisfied by the selection of a sufficient lube viscosity at the expected operational temp range.

Is that a better interpretation of your information?
 
That marketing piece with nonexistent grades has never meant much that I’ve relied upon. But maybe that’s just me.

There is marketing attached to it, but it's not without merit. A lot of data collection, engine builds and teardowns, and analysis went into that chart.

Ah ... I think I see what you're getting at.

Again, I'll nit-pick, but not meant as offense to you, just the way you stated it. If this comes off rude then I certainly apologize in advance; not meant as an affront to you personally.

If I understand you correctly, it would be better stated from your point of view in that bearing clearances are part of the determining factor in the selection of a viscosity grade. It's not that clearances determine the viscosity of a fluid, but to avoid damage, the clearances must be maintained by a lube which can provide a viscosity sufficient to hold the MOFT (often assured by grade or HTHS specs).

When you stated it previously (hence my initial objection), you said "viscosity is determined by 3 factors". I took that as the lubricant viscosity is manipulated by those three things (I do agree that temp directly affects it; the other two, no).

But what you are really trying to convey is that bearing loads and clearances dictate a specific grade for the acceptable minimum film barrier, etc. And that, in turn, is how the OE determines the viscosity recommendation. Your point became more clear when you said "bearings decide the viscosity".

It's not that bearing clearances actually affect the viscosity of the lube. Rather, the bearing clearances need a MOFT barrier, and that is criteria used to select a lubricant of grade "X" to satisfy that requirement. Viscosity of the lube isn't "determined" by those characteristics. Viscosity desired to maintain those clearances results in a selection of the recommended grade (or HTHS value). Bigger clearances dictate the need for thicker vis.

My view is that viscosity of the lube is determined by chemical composition and temperature. However, I can import your points into my view by agreeing with you, in that bearing clearances need a lube with a MOFT, and that is satisfied by the selection of a sufficient lube viscosity at the expected operational temp range.

Is that a better interpretation of your information?

Yes, that's what I was getting at. I should've worded it better. Your reply is making more sense to me now too.
 
There is marketing attached to it, but it's not without merit. A lot of data collection, engine builds and teardowns, and analysis went into that chart.
And after all that effort, they felt the need to insert some non-defined SAE grades into the chart? Using the units of measurement alone would have been a more straightforward approach.
 
And after all that effort, they felt the need to insert some non-defined SAE grades into the chart? Using the units of measurement alone would have been a more straightforward approach.

Driven is mostly racing oriented. They make oils in 0W-5 and 0W-10 for specific racing classes, hence their inclusion. Their target audience isn't someone on BITOG worrying about whether an oil grade is official or not.

Red Line, HPL, and LAT also make 0W-5 and 0W-10 oils. It's just something to reference the relative viscosity. Don't think too hard about it.
 
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I ran 10w-30 or 10w-40 in my Crown Vic and it went 375,000 miles. It would smoke a little after a stop. It did not die of engine failure. 1995 4.6L.
 
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