Oil - Another "why" question

[crazyal]

The dumbed down way I've always understood it (and I could be wrong so hopefully someone else will correct me) is that 5w40 oil is 5 weight oil that acts like 40 when at normal operating temps. To do this they add stuff to make the oil "thicker" than it really is. But those additives don't act in a linear fashion. So as the oil gets beyond it's normal range the oil doesn't continue to get "thicker". That's why your chart stops at a lower temp for the Xw30 weight oil vs the Xw40 weight oil. When the outside air temp is hotter it's assume that the internal oil temp will also be hotter. It may be a broad statement as things like oil coolers will help reduce the temps it's better to assume worse case than best case.

 

[Copperhead]

Actually, no. The number in front of the "w" is the winter flow rating and not the same as a weight rating. It is more about flow rate at a given temp. Weight rating (the number after the 'w") has to do with molecular composition and working kinematic viscosity. And it is a corresponding thing to High Temp High Shear (HTHS) rating. For instance, a typical 40w oil has a HTHS rating of around 4.5, whereas a typical 30w oil has a HTHS of around 3.7. Many confuse the cold flow rating with a weight rating.

There are some high end synthetics that can have a low cold flow rating and use no viscosity improvers to be the weight rating. A lot of high end 10w30 oils are literally a straight weight 30 oil that has a cold flow rating of 10.

 

[dragoneggs]

To understand the way outside temps affect an engine is really simple, just take the human body for instance, a marvelous engine in itself, it burns fuel to make heat and energy just like an internal combustion engine. SO take your human engine to the wood pile and start chopping wood when it is 40F and compare your body temp to what it is if chopping wood at 100F. You would see that you could run your human engine much longer at 40F without overheating than you can at 100F. There isn't much difference in our bodies from an engine, both require fuel, air and water for coolant and if you run out of either, your engine stops.

Hi Gary, not a bad analogy!

 

[Copperhead]

The problem with the analogy (for the purposes of the oil discussion here), is that an engine operates at a controlled sustained temperature range whether the outside temp is -40F or 110F. True, the body will cool via perspiration, but that eventually runs out without replenishment and is not as effective in high humidity. Engine cooling is a totally different concept. And on HD working engines, gas or diesel, the oil is kept at approximately the same temp as the coolant flowing thru the motor. Whether my Detroit Diesel has a 40w or a 30w oil in it, whether I am hauling something in MN in the dead of winter or in TN in the heat of summer, the temp of the oil is staying within a few degrees of the coolant temperature which is controlled by thermostat. Even the oil cooler has a thermostat. And since one of oil's primary tasks within an engine is cooling, generally a lighter viscosity oil will shed heat via the cooler a little better than a higher viscosity oil. So, the idea of the thicker viscosity for hot weather is negated.

The analogy might correspond better to a air cooled engine.

 

[deezler]

The problem with the analogy (for the purposes of the oil discussion here), is that an engine operates at a controlled sustained temperature range whether the outside temp is -40F or 110F. ......on HD working engines, gas or diesel, the oil is kept at approximately the same temp as the coolant flowing thru the motor.

On an HD diesel with separate oil cooling thermostat, fully engineered for constant towing thermal loads, yes, I see what you are saying. But in almost every automotive or pickup engine, the oil is going to be hotter when operating in summer ambient temps vs winter. It's that simple. The oil lines, filter housing, cooler, and even the block itself are all radiating heat away. They can do this less effectively in hotter ambient temperatures. So, the oil will be thinner. Thus, thicker oil recommended for hotter ambient temperature operation... nothing really to debate here.

 

[Gary Fowler]

I don't know anything about diesels so I can't comment on them. I thought that they produced higher shear loads. But it's pretty interesting what copperhead says. I was told by a very good mechanic that the engine bearing tolerances were tighter than they used to be due to improved manufacturing and so they needed lower viscosity oil to be able to flood the bearings.

I don't remember any auto manufacturers recommending Xw-40 oil since the 1980s. I didn't do a survey though. But all my modern vehicles, and small engines for that matter, say to use a lower viscosity. My 1993 Ford Windsor 5.8 has a sticker right on the motor that says to use 10w-30 oil. Of the six post 1990 vehicles I've owned, all have required lower viscosity than Xw-40. Spec'd out in the owners manual.

If 5w-40 is better than the other oils, then why don't any auto manufacturers recommend it?

I don't have data to prove my theory but I think the use of xxW-30 oils in newer cars is just so they can get another MPG out of the engine so they look good to the consumer and most importantly to the FEDS mandated higher mileage cars.
I don't think it has anything to do with bearing clearances. The oil molecule is still the same size regardless of the Weight of the oil. 5W-30 and 5W-40 will have the same cold start properties which is where 99.9% of your engine wear occurs.

 

[deezler]

That's absolutely right, Gary. At work we ran an engine on some 0W16 oil the other day for dyno testing. It was almost clear and poured like water. Kinda freaky pouring it into the engine, but the engine didn't seem to know the difference.

 

[GoldenCityMuse]

Yes, the low weight oils such as 0W20 are used mainly for the 0.01 MPG increases it produces. It might be 0.04 MPG for all I know, but they are trying everything in the book to increase CAFE.

 

[3930dave]

I don't have data to prove my theory but I think the use of xxW-30 oils in newer cars is just so they can get another MPG out of the engine so they look good to the consumer and most importantly to the FEDS mandated higher mileage cars.

Roger that.

I helped a lady select oil for her late model Mazda car a while back. She mentioned that she thought it need 5W30.... I had a look at her Owner's manual..... turns out that she had flipped to the page that listed the viscosity for use in Mexico.

USA spec for that car was 0W20, including Puerto Rico...... must be due to that night and day difference in climate between Puerto Rico and Mexico

Rgds, D.

 

[Copperhead]

On an HD diesel with separate oil cooling thermostat, fully engineered for constant towing thermal loads, yes, I see what you are saying. But in almost every automotive or pickup engine, the oil is going to be hotter when operating in summer ambient temps vs winter. It's that simple. The oil lines, filter housing, cooler, and even the block itself are all radiating heat away. They can do this less effectively in hotter ambient temperatures. So, the oil will be thinner. Thus, thicker oil recommended for hotter ambient temperature operation... nothing really to debate here.

Oil viscosity is determined at 100C. Viscosity is not thickness. Viscosity is technically friction. Yes, oil has friction, as do all liquids. Ask Isaac Newton in the next life and he'll let you in on it. The higher the viscosity, friction is also higher, and the heat generated by the friction is higher as well as the energy expended to move a heavier oil around the engine and overcome the higher friction. What is really telling about an oil is the High Temp High Shear (HTHS) rate that it has. A typical 30w can perform equally with a 40w if it's resistance to shear is good. I have seen 15w40w oils shear more than 10w30w oils in oil samples from engines under working loads. Most of what you are talking about was important back when there were only conventional oils on the market. Modern high end conventionals, syn blends, and full synthetics with the right add packs can be very resistant to shearing so that the old methodology of needing higher viscosity oils under high load and high heat has been effectively eliminated. Even racing engines are moving to lower viscosity oils because of that.

And taking this into account, yes, the ability for lower viscosity modern oils to meet the needs of engines has provided the means to also tweak out some better fuel efficiency. Lower viscosity, less friction means better fuel economy. The OEM's and the EPA are just taking advantage of the technology that makes up modern oils. But old habits die hard. There are still those that swear by 3000 mile oil changes too.