Oil viscosity question

[Flashpoint]

So originally there were single viscosity oils like 30W, etc. These oils were a certain viscosity (thickness) at a certain temperature and the viscosity varied directly with temperature, thicker when cold, thinner when hot.

Then came multi viscosity oils like 10-40W, etc. that attempt to maintain a constant viscosity throughout a temperature range by acting (for example) like a 10W when cold and a 40W when hot. So my question is what median viscosity is 10-40W oil trying to maintain? For example if I substitute 5-40W for 10-40W is it thinner across the entire temperature range or will it be the same but only thinner at cold temperatures?

 

[GymB]

I agree that oil viscosity is a curve relative to temp as you described, so 10W40 has a viscosity of 10 at say 80F and a viscosity of 40 at say 225F.

I think as engine tolerances got better thinner oils are required to properly lubricate thwm.

Does that answer your question?

 

[GoWolfpack]

There is an Engineering Explained video on YouTube about this.

 

[Flashpoint]

So no simple answer? It's basically a simple question:

If I substitute 5-40W for 10-40W is it thinner across the entire temperature range or will it be the same but only thinner at cold temperatures?

 

[Trevillian]

If I understand it correctly. The first number (5 or 10, etc) is the viscosity of the oil while cold down to a certain ambient temp making it easier for the engine to get lubed on initial start up.

Then once up to operating temp the oil becomes the next "number" listed for viscosity which is ideal for whatever particular engine during normal operation.

I.e. light viscosity to send the oil where it needs to be as soon as possible then as normal operating temps are achieved the oil transitions to the "ideal" thickness for that particular engines parameters for continuous operation.

 

[Trevillian]

So no simple answer? It's basically a simple question:

If I substitute 5-40W for 10-40W is it thinner across the entire temperature range or will it be the same but only thinner at cold temperatures?

Yes. On a basic level lol.

5w will be thinner on initial start up than 10w. But once normal operating temp occurs, they will both effectively operate as a 40 weight oil.

You can go down the rabbit hole of "startup/initial" viscosity but I've always just gone with what the engineers figured out.

I know of the back in the day "high mileage" gasoline pickup truck engines. If you switched to the Shell rotella 15w-40 for diesels. The old farm truck would just keep chugging. But it's beyond me why or if it had any benefit.

 

[GoWolfpack]

In the very beginning of the video he talks about oil weights.

 

[Mike79]

10w 40 will be 10 weight when cold and 40 when hot.
5w 40 will be 5 weight when cold and 40 when hot....to make it simple. This is not 100% true but us a simple straightforward answer.

 

[Get Off My Lawn]

If’n I recall the detailed study I read in the 80’s…
The meaning of 10W-40 is it flows as straight 10 weight oil at 0 degrees, W stands for winter. It flows as straight 40 weight oil at 200 degrees. To get the multi viscosity oil they added polymer molecules/strands to 10 weight oil to make it ‘thicker’ at high temperatures. The polymer additives are what also made the oil get dark so quickly when they got hot. The synthetic oils kept a consistent flow through extreme temperatures, so they used to be sold as straight 30, 40, etc weight.
I don’t know what they’re adding to the synthetics now.
The newer engines have much tighter tolerances that require thinner oils like 0W-20.
I would typically go from a 10W-30 to a 10W-40 when an engine has a lot of miles. The thicker oil will give more protection where there’s more of a gap from wear and tear.

 

[Beef15]

The two numbers are from different scales which are measured at different temps.
W in multi-viscosity oil does not stand for weight, it stands for winter. Based on sub freezing temp testing.
20W-50 and 20W are not going to be the same at the winter test temp.
5W-40 and 10W-40 should have a viscosity modifying package that will have them flowing very similar almost identical at the test temp typically 212. But 5W-40 will flow a little better at low temps.

 

[Flashpoint]

Thanks for all the replies. I always thought the lower number was for winter starts, as in the lower the first number the colder it can get and the oil will still be liquid. I remember trying to pour 30W at 6 degrees F, you better have a lot of time on your hands, lol. So the jist of my question was if I substitute 5W40 for an engine rated for 10W40 will the oil be thinner than desired at startup during non-extreme temps as we have here in NC, or will it essentially be the same?

 

[Supermatch]

If anything, the lower viscosity oils are better for reducing engine wear in cold start conditions, which is why you're seeing that bottom number trend to 0 weight oils in modern engines.

 

[Mike79]

Thanks for all the replies. I always thought the lower number was for winter starts, as in the lower the first number the colder it can get and the oil will still be liquid. I remember trying to pour 30W at 6 degrees F, you better have a lot of time on your hands, lol. So the jist of my question was if I substitute 5W40 for an engine rated for 10W40 will the oil be thinner than desired at startup during non-extreme temps as we have here in NC, or will it essentially be the same?

The simple answer to your question is no it will not be the same...
The manufacturer has that standard for a reason....now if it called for 5w40 and had 150k miles on it 10w40 might be OK due to the added clearance from wear....or it might not be.

Many moons ago when I work at a Ford dealership I'd see it ALOT! Customer comes in states my eng is making a knocking noise. Pull the car in it calls for 5w20 and the shinny new quick lube sticker has 10w40 on it. Explain to the customer they have the wrong oil...BS I've been told 95% of the time....change the oil and wow magic the variable cam timer is now getting correct oil pressure and it doesn't rattle anymore.

 

[Beef15]

Thanks for all the replies. I always thought the lower number was for winter starts, as in the lower the first number the colder it can get and the oil will still be liquid. I remember trying to pour 30W at 6 degrees F, you better have a lot of time on your hands, lol. So the jist of my question was if I substitute 5W40 for an engine rated for 10W40 will the oil be thinner than desired at startup during non-extreme temps as we have here in NC, or will it essentially be the same?

Not the same, thinner yes, not enough to worry over, your manual may well list it as an approved alternative or have a temp table where it's recommended, though those are much rarer these days for light duty vehicles.

 

[Diablos]

manufacturers have gone to thinner weight oils due to cafe standards as well

 

[Flashpoint]

Cool. Well last night my Yamaha FJR just got 5w40. It'd hard to find 10W40 synthetic sometimes, especially in 5 qt jugs. I could have gotten a diff brand in individual quarts but paid > 2x more.

New oil is a beautiful thing, lol.

 

[Mr.Gadget]

I know the Dodge and Ram motors have had a lot of problems due to oil.
It calls for one type and weight, people using the cheap oil change have the knocking all the time.
Neighbors Chevy was that way. He change with wrong oil. Few days later he had to replace the cam shaft and lifters.

 

[Flashpoint]

I know the Dodge and Ram motors have had a lot of problems due to oil.
It calls for one type and weight, people using the cheap oil change have the knocking all the time.
Neighbors Chevy was that way. He change with wrong oil. Few days later he had to replace the cam shaft and lifters.

Some vehicles today have extremely specific requirements. The FJR does not but it can't handle certain additives like friction modifiers due to the wet clutch I believe.

 

[Mr.Gadget]

Some vehicles today have extremely specific requirements. The FJR does not but it can't handle certain additives like friction modifiers due to the wet clutch I believe.

Yep same for my ATV
You add the high dollar oil to it and your clutch will cry out.....

It needs standard wet cluch motor oil.
I have always used the factory Honda oil and filters. It's not like it saves much h buying the cheap stuff and it will kill the clutch if you get the synthetic high dollar stuff.

 

[SPST]

Cool. Well last night my Yamaha FJR just got 5w40. It'd hard to find 10W40 synthetic sometimes, especially in 5 qt jugs. I could have gotten a diff brand in individual quarts but paid > 2x more.

New oil is a beautiful thing, lol.

View attachment 643667

I hope you are not using an automotive oil that has the "Energy Conserving" symbol on it? You need to have a JASO MA2 or MA1 rated oil in bikes that use the same oil in the engine and transmission. Energy Conserving non JASO rated oils can ruin your clutch plates.

 

[Flashpoint]

I hope you are not using an automotive oil that has the "Energy Conserving" symbol on it? You need to have a JASO MA2 or MA1 rated oil in bikes that use the same oil in the engine and transmission. Energy Conserving non JASO rated oils can ruin your clutch plates.

Yes I was aware of and checked for that. Thanks for having my back though.👍