I currently run the Ford 15w-40 oil all year round here in Phoenix in my 6.0 Diesel Excursion. Been wanting to switch to synthetic but what weight oil should I use in temps from 40F-120F degrees? Is 5W-40 Mobil 1 synthetic for Diesels OK? Should I be using a 15W-40 oil in this heat? Getting ready for a trip across the country this summer and will be towing heavy and want to run synthetic oil but am wondering which weight would be best for my conditions?

 

I would go with 5w-40 synthetic if you do a lot of short trips where the engine barely warms.

If you are doing long trips heavily loaded... I would go with 15w40 synth.

5w-40 is theoretically as good as 15w-40 at the top end, but in practice, the 15w-40 have slightly more top end head room by the spec sheets --- though you have to look at individual manufacturers as each is different.


If you are lazy.. buy one and stick with it.


I would go for the 5w.

 

If its 5W40 or 15W40 it doesnt matter, when the eng is warm its 40 for both of them when both oils are hot.

5W40 is a higher grade oil because preventing viscosity change is hard so the quality of the oil is higher with 5W40.

5w40 will be less restictive for cold starts, which will make the eng run easer when cold.

5W40 is a better choice over 15w40 for cold or hot weather.

 

The 5W40 syn. will also have less coking in the super hot turbo in those conditions. IE; engine shutdown.

 

Rotella 5w-40. All year round for me.

 

Whatever the outside temp is, the oil temp when warmed up will be in the 190-210 range. The thermostat regulates coolant temp and the oil is cooled by the coolant. So many people ask about oil weights in different operating temps and it really doesn't make a difference except for cold starts.

 

I dont know about diesels, dont have one, so that may change the requirements.

I am not buying into this argument that a 5W-40 is the same viscosity hot as a 15w-40. Yes, I have read the specs. Viscosity is Viscosity is Viscosity!! When the "standards" organizations added the "W"designation, they played (in my opinion) a symantics game on us. The "W" designation was added (according to my reading of the specs) as a designator to clearify the cold weather thinking and therefore caused some "symantics"

Viscosity is a rating independant of W designations. It is very simple, oil is rated as it flows through a predetermined orifice at a specified range of temps, from low to high. At the highest temp, it will be the thinnest and will flow at a specific rate. That is the lowest rating #. Within the temp range, it will become no thinner than that Number. Dosnt imply what is will do out of range. At the lowest temp it will be the thickest and flow at a specified rate and will not get thicker--within the temp range.

The symantics comes in when they throw a wrench into the standard by saying "It behaves like a different viscosity" and adding the W. I see it as a simple issue, if it behaves like a different viscosity, then it IS a different viscosity, per the standards. So, rate the viscosity number accordingly.

I will and do run 10W-30 in this florida heat. Totally insignificant at startup as a protection issue. A thinner oil will flow out of bearing surfaces quicker (twice as quick?) than the thicker. If you dont have oil protection for bearing surfaces, then your oil is NDG---get a different brand. Oils have a quality rating that relates to their ability to cling to surfaces. I buy oils that cling to my bearing surfaces until the system pumps more to them. My engines (particularly the 4.6 merc GM that I recently changed to 10W-30) do not rattle nearly as much on startup with the "10" as with the "5". Anyone stupid enough to crank an engine and take it straight to 4-5k rpm is too stupid to worry about oils anyway!!

I have seen the arquments by "engineers" on this board, and respect them---dont want to flame anyone, but I "aint" buying into this symantics game of 5W-** over 10W-**. In order for the symantics to be valid on viscosity, I would have to see credible evidence that the basic test to determine viscosity has changed.

I am going for popcorn-------------------

 

The lower viscosity cold rated 5w-40 helps on cold starts due to the HEUI injectors (oil fired injectors). True, a 15w may or may not provide slightly better protection due to the "clingyness" (technical term?). The synthetic 5w-40 will also not create as much coking in the turbo and injectors when turned off hot, so it has advantages there.

 

Now that sounds very reasonable

 

No popcorn needed - you are simply wrong.

Synthetic oils have viscosity modifiers that enable the viscosity to change with temperature - more so than conventinal oils. That is how synthetics are able to maintain proper viscosities over a wide range of temperatures.

Read up on it w/ an internet search. You will see tests and engineering standards. Read up on the definition of the #w-## rating.

When you offer a challenge without taking the time to even do simple research, all it is is an invitation to an arguement - not a technical discussion.

Hopefully this can get back to a "diesel" related discussion.

 

Well this is still not convincing me that 5w would be better than synthetic 15W oil here in AZ. I think I will go with 15W synthetic since the temps here are so insane in the summer! Towing my 11K up a 7% grade when it's 118 degrees is pretty hard on a motors cooling/intercooler/oil cooler system! I'm sure my oil temps are at there max during these times, but I guess the 5W might flow better at the higher temps than 15W? Jeez we need an oil engineer on here to give us the facts........... Hey, but its a dry 118 heat!

Anyways I don't know if this thread is educating me to the point that I can confidently choose one or the other? 15W has been working fine, so maybe I'll just switch from 15W Dino to 15W synthetic??

 

They are both 40w oils, they just have different viscosity modifiers in them to act as a 5w or 15w when cold. When hot, their viscosities are that of a 40w, look at their data sheets.

 

First - there are only two 15W-40 synthetics that I know of. Because of the additives, the MAJORITY of the synthetics have a wider range of viscosities.

There is a specific viscosity "range" assigned to each weight of oil - this is by scientific definition so that the industry could have a reference point. For that to mean anything, it has to be at an assigned temperature. That reference point is 100 degrees C for the "single grade oils". Single grade oils (by defnition) cannot contain VII's (Viscosity Index Improvers). Multi-grade oils can and do contain them. While many synthetics contain VII's, some have been engineered to not require them. These are the VERY expensive synthetics.

At 100 degrees C
0W oil - 3.8 - ??? (I don't have this number readily available, but it doesn't matter)
5W oil - 3.8 - 4.09
10 W oil - 4.1 - 5.59
15W oil - 5.6 - (I don't have this number readily available, but it doesn't matter)
30W oil - 9.3 to 12.5 cst
40W oil - 12.5 to 16.3 cst

The viscosity is defined as the number of seconds it takes for an oil to flow through the test equipment (a viscometer)

All of these oils will be MUCH thicker at 0 degrees F than they are at 100 *C. That means at cold temperatures a higher rated "winter" oil (ie 15W40) will not flow as well as a 5W40 - period! This is by definition and verified through physical measurement and testing.

For multi grade oils, the number before the "zero" is how that oil behaves in cold weather oe "W"inter (most commonly referenced to be at 0 *F). So ............. a 5W40 oil has the viscosity that a 5 weight oil does at 0 *F and a viscosity of a 40 weight oil at 100 *C. This is the S.A.E (Society of automotive ENgineers) definition.

For everyone that has a diesel truck, your engine oil temp will run around 200-220 degrees F (and 100*C is apprx 212 *F). A 15W40 will run at the same viscosity (within a few percentages anyway) as a 5W40 oil - BY DEFINITION.

Unless the dry heat of Arizona will make a diesel engine run hotter than other places (which it doesn't do so by much), the engine temperatures will be about the same. The heat brought into an engine from the hot air is minimal compared to the heat released from combustion. The radiator system is designed to try to hold around 190 degrees - relatively independent of the surrounding temperatures (in America anyway). The radiator is quite large enough and the thermostat design is adequate to do this in both 120 ambient conditions or in cold weather.

When an oil is cold, the viscosity will be higher - no matter what oil and no matter what the grade. Remember, regardless of the rating of the oil, it gets thicker when it is colder. A 5W40 oil will behave loke a 5W oil when the temperature is at 0 *F while a 15W40 oil will behave like a 15W oil. Remember - this is at ZERO degrees F.

At 40 degrees C (apprx 100 degrees F) - most 15W40 oils will have a viscosity of around 100 to 120 cst (Shell Rotella 1W40 is 120). Remember the 40W rating at 100 degrees C (it is around 12 to 16). Think about how much thicker it will be at 0 *F - remember the viscosity definition is "flow time through the test equipment".

At 40 degrees C (apprx 100 degrees F) - 5W40 oils can have a viscosity of between 50 and 100 cst (regular Shell Rotella 5W40 is 95, the Rotella T6 is 87).

The viscosity differences are MUCH wider when you get to 0*F. Then you need to be looking at the pour point (which is the temperature at which the surface of the fluid does not move for 5 seconds when trying to "pour it" .... or something like that). Remember, our engines will all operate in a narrow range, but they start up in a WIDE temperature range (-20*F to 120*F). Ford tells us specifically not to use a 15W40 oil at low temps, but they tell us we are fine to use a 5W40 at high temps.

References

Fil's Auto Corner: Oil Facts v1.2

and

Motor Oil Viscosity Grades Explained in Layman's Terms

and

http://www.audiworld.com/tech/eng14.shtml

and

http://www.answers.com/topic/motor-oil-2

http://www.bobistheoilguy.com/visc.html

http://mysite.verizon.net/oldhokie/windyridge/oil.pdf


Edit - more info:

The standard unit used to measure viscosity is the centistoke (cSt). According to the Automotive and Industrial Lubricants Glossary of Terms:

Viscosity is ordinarily expressed in terms of the time required for a standard quantity of the fluid at a certain temperature to flow through a standard orifice. The higher the value, the more viscous the fluid. Since viscosity varies inversely with temperature, its value is meaningless unless accompanied by the temperature at which it is determined. With petroleum oils, viscosity is now commonly reported in centistokes (cSt), measured at either 40°C or 100 °C (ASTM Method D445 - Kinematic Viscosity).

 

I use Synthetic 5w-40 here in Las Vegas year round. I won't use any other kind of oil. I was using Rotella but Wal-Mart didn't have Rotella in the gallon jugs last time I went there instead they had Mobile 1 5w-40... I think my truck actually likes Mobile 1 more.

 

As you acknowledged, diesels have totally different requirements and are separately rated by API using the "C" series, i.e. CJ-4 instead of the "S" series like SJ.

Simply put - it is very difficult to get a gasoline engine oil to work well in a diesel.

Requirements are different, though many good diesel oils will work well in gasoline engines (with very good "S" ratings).



The issue you raise of oil adhering to surfaces has been exhaustively investigated by tribologists.

Adhesion is an issue until other issues become more important:


A) At extreme cold temperatures below its "pour point", adhesion becomes irrelevant as the oil turn to a wax / solid.

That solid film (whatever is left) have quite different properties from the oil film and is generally acknowledged to have considerably poorer lubrication qualities.


That is why in extreme cold, differentials that are not pressure lubricated (nearly all) will "heat up" in extreme cold and essentially, melt the heavy oils around them until they become normal.

If the lubricant was working great, there would be very little heat generated once the grease like oils are pushed out of the way.

The fact is, there is --- from lack of proper lubrication by the "cling on" oil film.

Hence the heat generated.

If you have a better idea of where the heat comes from --- do tell us.



B) The second circumstance where "cling on" oil is all that is doing the job is in the time when the lubrication system is not "pumping" to the point when oil pressure is "normal", and when pumped oil have a chance to flow to the highest points in the engine --- normally the valves and cams (for an overhead cam).


Mobile 1 engineers did extensive testing of exactly how much wear is done by the engine while it works on the "clinging film" vs. pressurized oil feed.

Their overwhelming conclusion is that the vast majority of engine wear occur in the first 30 seconds (in temperate climates) when the engine oil pump is not delivering full pressure and oil volume to every single nook and cranny of the engine.

In colder climates, an engine can be without lubrication in many parts for up to several minutes before the oil in the sump pumps or flows.

It is possible for old dino oil to be so frozen in extreme cold that it is virtually impossible to crank a motor without warming the motor with heat first.


The amount of lubrication done by what oil "clings" is minimal compared to a regular, fresh supply of oil.



That is not so far fetched ---- since one of the function of pressurized oil is to ensure that a large amount of fresh oil is always at the friction surfaces.

The "flush" ensures that wear particles, dirt, and debris that is normal from any engine and created / shed as part of normal machinery operation is flushed away by clean, filtered oil.

No matter how good an oil "clings", it simply cannot flush away the wear particles and dirt.


That is why parts that are greased never quite last as long as parts that are bathed in an identical weight of clean, filtered, oil.

There is no place for the "crud" to go, and it is the crud that wears out the parts.

Hence, ball joints and other greased parts last a lot longer if you do a "grease job" properly --- that is to say, over fill the grease so that the older grease is forced out of the joint by fresh grease, and in doing so, remove the debris, particles, dust, dirt, etc. that is doing the wearing.

In fact, serious heavy duty trucks do not rely on grease for things like ball joints, but a system of forced oil lubrication that forces heavy weight oil / grease into things like ball joints under pressure --- and by doing so, extend the life of these joints by at least 2X over a regular greased joint.


The advantage of clean, pressurized oil vs. the "cling film" is generally acknowledged by tribologists and conventional wisdom among the SAE.

For these reasons, automobile engineers have overwhelmingly switched to 5w oils at the cold end --- because there is so little wear that happens once an engine warms that require higher weight oils at the warm end. The goal is to reduce wear where 90% of it happens --- in the first minutes of a startup.

Mobile 1 and other auto engineers scientifically documented this with test engines that show exactly how much difference there is using 5w vs. a heavier weight "cold" oil.

If you are an SAE member, you can get the research publications (they are about 10 to 15 years back) for free.


Go look at any modern car built in the last 5 years, and you will find that the standard grade is 5w-xx, regardless of how hot it is operated in.

Some manufacturers, notably Ford, have gone so far as to use a lighter oil at the warm end, e.g. 5w-20 in nearly their entire line of gasoline engines. The reason is the 20 weight oil do the job at "hot", and the thinner oil gives a slight edge in fuel economy.

That is not applicable in diesels, who use oil to power fuel injectors that require extremely high pressures, and hence, for diesels, they retain the recommendation for XXw-40 weight at the warm end.

You will note that very few modern oils rated for diesels can deliver a wide range of viscosity for XXw-40.

Once you are into 5w-40, that is primarily the province of pricey synthetics because it is very hard to make a dino oil achieve that range and hold diesel specs.



So, as for your contention that 10w-30 appear to work better for you.... well, if you have an old engine, that is possible --- especially if you have a weak oil pump and maybe constricted (sludged) oil galleys.

But if you are starting from new --- 5w-xx (whatever the manufacturer recommends) is the way to go.


FYI, I have operated vehicles in extreme heat with heavy loads --- and I have never had a problem with the performance of oils at the "hot" end even when it is only 20 weight (e.g. my Ford gasser 5.4 loves 5w-20).