Oil Viscosity--Food for Thought or Fuel for a Flame War?

[CalG]

It’s not uncommon for people working with diesel equipment to dump 15w-40 in their gas pickups and it’s been fairly well documented that Ford 5.4s or v-10s don’t like that.

an explanation of the failure would be in order.

"Not liking" is rather vague.

True though, there is a popular misconception that "heavy weight oil is "better" for compression ignition IC engines. I can't figure where that has come from. "More is better"?

The hand book for the K series Kohler engines suggests "straight 30" is the best ...in all conditions. Using multi vis oil can cause excessive oil consumption! Go figure.

 

[North to Alaska]

If it 20 degrees Fahrenheit….don’t worry about. If it’s -10 to -40 degrees Fahrenheit like here in Alaska…worry about it.

a 10w or 15w at -20f is not lubricating your engine because it is nothing but a gel at those temperatures. The qualities of the oil that provide lubrication within those tight tolerances is not there at those temperatures within those viscosity grades. Furthermore, it will take a while for the oil to warm up enough to even be pumped efficiently to provide more lubrication.

I’ve used 15w40, 5w40, and 0w40.
A 15w40 will have a difficult time even starting due to the resistance between moving parts in the engine.

 

[Williy]

I read some place that regular oil will drain off
from engine parts so with regular oil bare metal
on metal but the synthetic leaves a thin film on
the metal parts. Anyone else read or hear this??

willy

 

[MHarryE]

An illustration. Journal bearing. Modern technology allows manufacturers to reduce the gap between bearing and journal. However, thinner oil is needed. That’s why new cars recommend oils like 5W-20 and the like. Diesels not so much and 15W-40 is almost a standard in warm temps. However, my neighbor got a new New Holland tractor and had an oil leak at the rear main during first winter. New Holland told him never run 15W-40 year around. 5W-40 in winter or year around. My Kubota/New Holland dealer stocks all the book recommended oils but the guys in the shop tell me the real story. Engines get synthetic unless the customer objects. My new compact track loader has synthetic gear oil in its finals instead of the book 80W-90 due to what they see when tearing down. Probably y’all down south don’t see a big difference but we northerners where our tractors have been started at temps below -30 many times this winter survive on big brand name synthetics. My cousin insists on the cheaper 15W-40 and is on his third rear crank seal at 3000 hours.

 

[rScotty]

I, too, follow the manufacturer's recommendations. I can't say that I've ever had a failure that can be traced to the oil. I've had a failure due to lack of oil! But not due to the oil itself.

Back when I had a mechanical shop you wouldn't believe the range type of oil in cars, trucks, tractors, and bikes that people brought in. Many times we saw vehicles with oil that hadn't been changed in 20 years or longer. It wasn't unusual to have upwards to half an inch of dirt and metal sludge in engine oil pans. I've seen old oily sludge so thick that the oil pan wouldn't drain. We would drive a punch through the goo to get the oil to drain, drop the pan, and clean the sludge out with paint scrapers.

Again and again we saw complete engine and running gear failures due to not having any oil at all in the engine, tranny, rear end, axles...etc.... . BUT,
I NEVER ONCE SAW A FAILURE THAT I COULD SAY FOR SURE WAS DUE TO BAD OIL or OLD OIL - or even to cheap oil.
Not once. No matter how bad or old it was or how hard it had been run or how burnt it smelled. I didn't even see the expected wear.

In spite of that, I put the best premium oil in my machines and always have. I haven't had any oil related problems either....but my experience makes me wonder.

rScotty

 

[LD1]

FWIW, 15w40, 5w40, and 0w40 are all the same viscosity oil.

The 40 is when the engine is at operating temperature. And there is no difference in the above three in terms of viscosity when running.

ALL oil thickens as it gets cold. A straight 40w motor oil would be EXTREMELY thick when cold. Thus the multi-viscosity oils.

Engines are designed around their operating temperature oil viscosity. So when the engine is cold and the oil is thick it isnt a good thing.

The multiviscosity oils are a misconception to many. 15w40 doesnt mean its 15 when cold and 40 when hot. That would mean it thickens as it warms and thins as it cools....which is far from the truth.

In reality....the 15 is meant as a comparison. It IS a 40w oil.....but it flows as if it were a straight 15w when cold.

So think of it this way.....with them all being 40w oils when warm......the 15w is gonna thicken up quite a bit when cold. The 5w wont thicken up as much, and the 0w even less so.

IF there was an oil that could maintain a CONSTANT viscosity across -40°F to +250°F that would be ideal. But that hasnt happened yet. They get close with the 0w40 and 5w40.....which are much better year round than 15w40.

Just cannot seem to get through to some people that insist a 15w40 is a heavier/thicker viscosity oil and 5w40 is a thinner viscosity oil....when it is not.

 

[Bill Guenthner]

Observation has shown me that oil clings to vertical surfaces and that time and the inexorable force of gravity causes that coating to thin. Thus I believe that the hardest part of an engine’s life is at start-up before the lubrication system renews the oil coatings.

It seems reasonable that, since modern engines are built to closer tolerances with narrower spaces between parts, thinner, I.e. lower viscosity, oils are desirable to provide lubrication in narrow spaces at low temperatures.

My tractor engine was designed some years ago, and 15W-40 is said to be good for all temperatures. The owner’s manual specifies:

Temperature​	SAE Viscosity​
Above 25°C (77°F)	30​	10W-30​	15W-40​
0 – 25°C	20​	10W 30​	15W-40​
Below 0°C (32°F)	10W​	10W-30​	15W-40​

Lower viscosity winter oils, e.g. 5W had been around for decades.

Now that 0W-40 and 5W-40 oils are available, many on this forum argue that the lower viscosities will lube better at low temperatures, especially at start-up when the engine is cold, and recommend their use.

Could it be that the higher viscosity oils cling more thickly and persistently and thus provide adequate lubrication to the older engines for which they were recommended? Also, might the lower viscosity oils thin too much in the wider spaces?

Just use what the owner’s manual calls for.

 

[mo1]

FWIW, 15w40, 5w40, and 0w40 are all the same viscosity oil.

The 40 is when the engine is at operating temperature. And there is no difference in the above three in terms of viscosity when running.

ALL oil thickens as it gets cold. A straight 40w motor oil would be EXTREMELY thick when cold. Thus the multi-viscosity oils.

Engines are designed around their operating temperature oil viscosity. So when the engine is cold and the oil is thick it isnt a good thing.

The multiviscosity oils are a misconception to many. 15w40 doesnt mean its 15 when cold and 40 when hot. That would mean it thickens as it warms and thins as it cools....which is far from the truth.

In reality....the 15 is meant as a comparison. It IS a 40w oil.....but it flows as if it were a straight 15w when cold.

So think of it this way.....with them all being 40w oils when warm......the 15w is gonna thicken up quite a bit when cold. The 5w wont thicken up as much, and the 0w even less so.

IF there was an oil that could maintain a CONSTANT viscosity across -40°F to +250°F that would be ideal. But that hasnt happened yet. They get close with the 0w40 and 5w40.....which are much better year round than 15w40.

Just cannot seem to get through to some people that insist a 15w40 is a heavier/thicker viscosity oil and 5w40 is a thinner viscosity oil....when it is not.

The viscosity ratings are a range. There are generally very slight differences in viscosity in a manufacturer's 0W-40, 5W-40, 10W-40, and 15W-40 (which are all 40 "hot" weight oils) at 100 C and 150 C which are the two "hot" temperatures the American Petroleum Institute (API) tests motor oils at. The differences are modest, but they are there, particularly in the 150 C High Temp High Shear test where there may be as much as a 10-20% difference in viscosity between a 0W-40 and a 15W-40. Some of the very high viscosity index oils such as 0W-40 may have a 150 C viscosity barely above that of a thicker 10W-30's 150 C viscosity as the viscosities are a range, and the 30 range directly abuts the 40 range. Whether or not this actually means anything depends on just how hard the engine is pushed, for how long, and how hot it is outside.

The cold weights are tested at a variable temperature below -20 C or so (varies based on weight) and at 40 C. The viscosities are indeed a lot higher for an 10W-40 at 40 C than at 100 C, the rating is done as a 10W-40 when cold is going to be similar cold to a straight 10W oil and generally similar to a straight 40 weight oil when hot. The 10W is going to be a whole lot thinner than a 10W-40 when hot, and a straight 40 weight is going to be far thicker than a 10W-40 when cold. Even an 0W-40 is at least 500 times thicker at -40 F than at 250 F.

 

[RalphVa]

Observation has shown me that oil clings to vertical surfaces and that time and the inexorable force of gravity causes that coating to thin. Thus I believe that the hardest part of an engine’s life is at start-up before the lubrication system renews the oil coatings.

It seems reasonable that, since modern engines are built to closer tolerances with narrower spaces between parts, thinner, I.e. lower viscosity, oils are desirable to provide lubrication in narrow spaces at low temperatures.

My tractor engine was designed some years ago, and 15W-40 is said to be good for all temperatures. The owner’s manual specifies:

Temperature​	SAE Viscosity​
Above 25°C (77°F)	30​	10W-30​	15W-40​
0 – 25°C	20​	10W 30​	15W-40​
Below 0°C (32°F)	10W​	10W-30​	15W-40​

Lower viscosity winter oils, e.g. 5W had been around for decades.

Now that 0W-40 and 5W-40 oils are available, many on this forum argue that the lower viscosities will lube better at low temperatures, especially at start-up when the engine is cold, and recommend their use.

Could it be that the higher viscosity oils cling more thickly and persistently and thus provide adequate lubrication to the older engines for which they were recommended? Also, might the lower viscosity oils thin too much in the wider spaces?

All water cooled engines operate with coolant set by a thermostat, often as high as 190-195 F. Therefore, those upper temperatures they mention absolutely do not make sense. If that 20w oil is good for 0 C, then it's good for outside temps as high as Gobi Desert hot because it would still be below 190-195 F. There are millions of VW diesels running around the world with 5w30 Castrol in them. None have ever suffered an engine failure due to lubrication.

They need to put transparent valve covers on all engines. Then, folks using 15w40 could watch their valve train being smoked before it got oil to lubricate it during a cold start.

Also, almost any oil will allow the engine to crank about 60% faster than a 15w40 starting cold.

0w, 5w and 10w renditions of 30 or 40 weight oils are better than the base ones. They'll still be the same 30 or 40 weight when hot but will actually be HIGHER vis than their base oil when hot. For instance, a 10w40 will be about 1 cSt high vis at 210-230 F operating temperature than a 15w40.

 

[Jerry/MT]

Observation has shown me that oil clings to vertical surfaces and that time and the inexorable force of gravity causes that coating to thin. Thus I believe that the hardest part of an engine’s life is at start-up before the lubrication system renews the oil coatings.

It seems reasonable that, since modern engines are built to closer tolerances with narrower spaces between parts, thinner, I.e. lower viscosity, oils are desirable to provide lubrication in narrow spaces at low temperatures.

My tractor engine was designed some years ago, and 15W-40 is said to be good for all temperatures. The owner’s manual specifies:

Temperature​	SAE Viscosity​
Above 25°C (77°F)	30​	10W-30​	15W-40​
0 – 25°C	20​	10W 30​	15W-40​
Below 0°C (32°F)	10W​	10W-30​	15W-40​

Lower viscosity winter oils, e.g. 5W had been around for decades.

Now that 0W-40 and 5W-40 oils are available, many on this forum argue that the lower viscosities will lube better at low temperatures, especially at start-up when the engine is cold, and recommend their use.

Could it be that the higher viscosity oils cling more thickly and persistently and thus provide adequate lubrication to the older engines for which they were recommended? Also, might the lower viscosity oils thin too much in the wider spaces?

For high temperature operating protection and thermal breakdown, the second number in the SAE grade is what is important ( i.e. 15W40). For easy cold weather starting and initial lubrication, the first number is the import one (i.e. 15W40). In really cold weather 5W40 would be acceptable in an engine that calls for 15W40.