This isn't an oil comparison debate.. I'll get that squared away right now .

I want to explore the different aspects of engine oil and how they effect the engine. I don't understand a lot of things about this, hardly grasp the weight thing. They say heavier weight oil protects the engine more, yet then they say thinner oil gets in places the heavier weight can't because of the smaller molecules. So I plan on ending all doubt in this thread.

Here's some stuff from my diesel book.

To start off, oil has 4 main functions in the engine:

[*]Lubrication: To minimize friction and act as a medium to support the hydrodynamic suspension of the crankshaft and camshaft.

[*]Sealant: Enables the rings to seal compression and combustion gases from the crankcase.

[*]Coolant: Heat generated by combustion and friction must be dissipated to atmosphere with heat exchangers (oil cooler).

[*]Cleaning agent: Condensed by-products of combustion hases end up in the engine crankcase and can combine to form harmful liquids (acids) and particulates (sludge).

Hydrodynamic suspension is what they call floating of engine components due to the film of oil between them. As in, the crankshaft does not sit on the bearing, it sits on a film of oil that is constantly being renewed thanks to the oil pump. The thickness of this film depends on the oil pressure and oil leakage rate back into the oil pan.

Now it's getting interesting.

The viscosity rating of an oil usually describes its resistance to flow. High viscosity oils have molecules with greater cohesion ability. However, properly defined, viscosity denotes resistance to shear. When two moving components are separated by engine oil, the lamina (portion of oil film closest to each metal surface) on each moving component should have the least fluid velocity while the fluid in the center has the greatest fluid velocity. Shear occurs when the lamina fluid velocity is such that it is no longer capable of adhering to the surface of the moving components. When we use the term friction bearings to describe crankshaft main bearings, the friction being referred to is the fluid friction of the lube hydrodynamics in supporting the crankshaft.

Two oils with identical viscosity grades can possess different lubricity. The lubricity of an oil properly describes its flow characteristics. Lubricity is also affected by temperature: Hotter oils flow more readily, colder oils less readily. In comparing two engine oils, the one that has the lowest frictional resistance to flow can be said to possess the greater lubricity. In thick film fluid lubrication, flow friction is determined by the fluid's viscosity, that is, its resistance to shear. In thin film or boundary lubrication, flow friction is determined by the lubricity of the fluid.

Still have no idea why a subzero engine would use 0W and a hot engine uses heavier weight. It said the one with the lowest frictional resistance to flow has more lubricity, so why not run 0W all the time? When I read this I saw a lot of conflicting info..

I am not a lubrication expert, but I have dealt with a lot of fluid mechanics in my career...

Something that you have to keep in mind is the bearing tolerances and the level of precision that the engine was designed to.

From what I read, lubricity is a calculated value, so I am going to ignore it for now. ha!

Viscosity though, as a resistance to shear, makes good sense.

In a bearing, say a crankshaft journal bearing, the primary purpose of oil is to keep the bearing surface from touching the crankshaft. Conventional wisdom says they will touch right when the engine starts before oil pressure gets up. As long as the engine isn't revved too high before the oil circulates, thats okay since the bearings are cool and it won't rub enough to get hot, soften, and gall the surface.

Shear is a function of the fluid properties and the surfaces that it is running between. The relative speed, surface area, and distance apart of the two surfaces is key. The thicker the fluid, the higher shear, the more pressure it will withstand. However, if the oil is too thick, it won't flow fast enough to make it into the bearings and it will cause metal to metal to contact and gall your bearings as well.

Shear also creates a lot of friction, so people can get better economy with thinner oils. However, in order to keep the bearings from touching with thin oils, they need to have the two surfaces (bearing and journal) closer together, in order to get the shear high enough to withstand the same loads. Ford has done this with their PowerStrokes, as they call for some very thin oil, especially for a diesel.

Ford is taking a big risk with their engines for the sake of economy. When you tighten up those bearing tolerances, you really reduce the size of particle (dirt, soot, metal chip...) that can safely be in your oil. Diesels usually are pretty dirty inside, as is evidenced by our oil turning black so quickly. Keeping up on oil changes isn't such a big deal on our heavy oiled old school style cummins ISB's. Fords though, I would probably change the oil every 2000 if I wanted to keep it. (But that would be silly, since its not a cummins )

The old school diesels like in my familys 1970s John Deere industrial equipment call for some very thick oil, something like 50wt and 100wt if I recall correctly. The cool thing about this is, that John Deere knew there was probably going to be DIRT in the engine, and it might miss an oil change or two. They still wanted it to run like a Deere, so they gave it some heavy oil and bearing dimensions that you could safely float some pea gravel through.

my 2cents

Gassers must have a tighter tolerance and since they don't get as dirty as a diesel, they don't have as much worry about particulates getting in between the bearings. Explain why cold engines need thin oil then. Is it because straight 40 weight oil will be too thick at subzero temps to get into the bearings and do any supporting, so you sacrifice a little viscosity so that oil can get in there, albeit not the right viscosity to properly support the components but it is better than having thicker oil that can hardly get in there? Is 15 weight oil the same viscosity at 0F as 40 weight at 200F?

Gassers must have a tighter tolerance and since they don't get as dirty as a diesel, they don't have as much worry about particulates getting in between the bearings.

Explain why cold engines need thin oil then. Is it because straight 40 weight oil will be too thick at subzero temps to get into the bearings and do any supporting, so you sacrifice a little viscosity so that oil can get in there, albeit not the right viscosity to properly support the components but it is better than having thicker oil that can hardly get in there? Is 15 weight oil the same viscosity at 0F as 40 weight at 200F?

Let me get back to you later on that... I gotta get some work done.

Yeah, gassers are usually a lot cleaner. The other thing to keep in mind with gas engines is that they have to run at a lot higher RPM than our diesels. Gassers also don't have 17:1 compression ratios and they can make the same power with a lot less torque, albeit at a lot higher RPM.

Bearing design can be pretty complex. I am going to see if I can swing by my storage unit tomorrow and grab some of my machine design texts... mabe that might help us. I honestly haven't looked at this stuff for years. This will give me something else to look at when I am stuck in a hotel this weekend.

Edit... Interesting article. I am having fun researching some of this!

http://www.upmpg.com/tech_articles/motoroil_viscosity/

I still want to verify from my books, but according to a quick search, 0W oil should maintain its viscosity per spec at -35°C, a 5W at -30°C and a 10W is tested at -25°C.

--- Update to the previous post...

Gassers must have a tighter tolerance and since they don't get as dirty as a diesel, they don't have as much worry about particulates getting in between the bearings.

Explain why cold engines need thin oil then. Is it because straight 40 weight oil will be too thick at subzero temps to get into the bearings and do any supporting, so you sacrifice a little viscosity so that oil can get in there, albeit not the right viscosity to properly support the components but it is better than having thicker oil that can hardly get in there? Is 15 weight oil the same viscosity at 0F as 40 weight at 200F?

Cold engines need thin oil because the oil needs to flow. The oil is pumped from the positive displacement oil pump, but it also needs to flow through the passages, the pickup, and into certain nooks & crannies. The oiling galleries are sized to get the oil around at the "normal" viscosity.

Basically, in a 10W-30 oil, starts out as a lighter oil that will meet the W winter spec, but has viscosity modifiers to keep the oil from thinning as it heats up. I am sure that there are a lot of hocus pocus additives for viscosity, but lets keep it simple with that model. The downside of these modifiers, is that they deteriorate and allow the oil to thin out at temperature. When I think about this, it passes the real world test. As when I do a hot oil change after a long period, the oil feels less thick than when new. Not scientific, but hey...

Is 15 weight oil the same viscosity at 0F as 40 weight at 200F?

I have no idea... I have looked, but haven't found any viscosity charts for oil vs temperature. I am willing to say that a 10W-40 is going to be the same viscosity at 0F and 200F, and a 0W-15 oil is going to also be the same at 0F and 200F.

From what I have read sofar, the standard for specifying the SAE # of an oil is at ~210F/100C. Since this is near the normal operating temperature for most engines, I am not optimistic that I will find many viscosity vs temperature unless I get into the lab and make some myself.

I think this website below will do a better job at outlining the issues than I can. Great page.

http://www.upmpg.com/tech_articles/motoroil_viscosity/

Another good site.

http://www.aa1car.com/library/oil_viscosity.htm

ISX, I was home recently and I couldn't find my books... I hopefully have them packed somewhere, but I can't seem to find any of my reference stuff. Sorry man.

what i know about this would fill up a thimble compared to what yall have already discussed. what i was told many years ago was the multi weight motor oils, using 10/40 as an example, would act like a 10 weight when it was cold and a 40 weight when it was hot. this allowed you to run it all year in most weather conditions. Please dont tell me what i have believed all these years is wrong. it always made sense to me.

what i know about this would fill up a thimble compared to what yall have already discussed. what i was told many years ago was the multi weight motor oils, using 10/40 as an example, would act like a 10 weight when it was cold and a 40 weight when it was hot. this allowed you to run it all year in most weather conditions. Please dont tell me what i have believed all these years is wrong. it always made sense to me.

I'm with you dripley, the understanding I've had for years was the oil viscosity was basically the same, say 5w-40 meant 40 weight but when the temperature was cooler, it flowed the same as a 5w oil, but still stay a 40 weight oil. The 'temperature' meant not just the atmosphere, but the internal temperature of the engine, the parts, the fluids within as the oil and such. So the oil is a 40 weight, but will flow as a 5 weight until the temperature increases and it changes flow characteristics as temp increases. It's hell getting old ..... I'm starting to get confused again.

a 10w40 flows as a 10w would flow when cold, and flows as a 40w would flow at operating temp. The only thing that matters is the temperature of the actual oil. Here's a good primer. http://www.blackstone-labs.com/oil-viscosity.php

speaking for me and i hope for anoldbiker, i am glad tht the primer you posted was short and to the point, and you did not destroy what i have believed most of my adult life. i am not sure i could have stood that. i do enjoy the technical posts that are on here and try to read and absorb them. but like anoldbiker, sometimes i get confused. please dont stop putting the info out there because i know it helps alot of people. i'm sorry:doh:, i think i am getting confused again. ps ISX. hows the back?

I'll try and keep this section active. I get bummed out after waking up at 5 to go to work, then getting back at 5 just to go to bed (try to) at 8 or 9. BUT!, should be over with. I have been thinking of some things to post in here just haven't done it. Back is fine, it's the shoulder I messed up Went riding last weekend so all is well. Going again this weekend.

I'll try and keep this section active. I get bummed out after waking up at 5 to go to work, then getting back at 5 just to go to bed (try to) at 8 or 9. BUT!, should be over with. I have been thinking of some things to post in here just haven't done it. Back is fine, it's the shoulder I messed up Went riding last weekend so all is well. Going again this weekend.

see i'm getting confused again. glad your doing good though.

a 10w40 flows as a 10w would flow when cold, and flows as a 40w would flow at operating temp. The only thing that matters is the temperature of the actual oil. Here's a good primer. http://www.blackstone-labs.com/oil-viscosity.php

I tend to enjoy this parting phrase from your link...

People selling oils and additives may be sincere, but they don't have to live with the results. They simply smile a lot on the way to the bank.

Now a couple weeks ago I had the Royal Purple rep in our NAPA store and he was telling me that the first # of the oil is the cold pour point and the second # was the operating temp flow point. Now as was said in this post not all 15w 40's are equal for instance they use less of the chemical that prevents sheer, but they use a product that keeps more of this product in the places sheer happens. Also another thing he said was that their oil is polerized so the oil accually sticks to metal. he said a good way to test this is you can't just wipe this oil off metal you have to use a solvent like brake clean to remove it. I hope I did not confuse any one any more but thought you would like to know:2cents: Thanks