The Naked Scientists

The Naked Scientists Forum

Author Topic: What oil viscosity is best for a car engine?  (Read 24143 times)

24 blacberry road

  • Guest
What oil viscosity is best for a car engine?
« on: 23/05/2011 17:30:02 »
24 blacberry road asked the Naked Scientists:
   
What oil viscosity is best for a car engine?

What do you think?
« Last Edit: 23/05/2011 17:30:02 by _system »


 

Offline CliffordK

  • Neilep Level Member
  • ******
  • Posts: 6321
  • Thanked: 3 times
  • Site Moderator
    • View Profile
What oil viscosity is best for a car engine?
« Reply #1 on: 23/05/2011 19:40:36 »
The lighter the viscosity, the better the oil...  up to a point.
Some new car engines specify something like 5w20.

However, there is a trade-off between low oil viscosity and oil consumption, and I think the 5w20 is too light. 

My preferred grade is 10w30 for a gasoline engine.
And, I think 15w40 for a Diesel engine.

Perhaps one should consider the age of the vehicle and oil consumption as part of the equation.

New car (gasoline): 5w20
100K+ Miles (gasoline), or after oil consumption problems: 10w30
If oil consumption problems persist: 10w40
 

Offline graham.d

  • Neilep Level Member
  • ******
  • Posts: 2208
    • View Profile
What oil viscosity is best for a car engine?
« Reply #2 on: 24/05/2011 08:52:36 »
Many synthetic oils that are recommended today 5w30 or even 5w40
 

Offline CZARCAR

  • Hero Member
  • *****
  • Posts: 686
    • View Profile
What oil viscosity is best for a car engine?
« Reply #3 on: 24/05/2011 14:04:50 »
i dont get why the oil is different, seems 5-40 would cover all the bases?
 

Offline Geezer

  • Neilep Level Member
  • ******
  • Posts: 8328
  • "Vive la résistance!"
    • View Profile
What oil viscosity is best for a car engine?
« Reply #4 on: 24/05/2011 17:14:53 »
It's a compromise. Ideally, you want an oil that has constant viscosity so that it always provides the same lubrication regardless of temperature. Unfortunately, the viscosity of oil does vary with temperature.

Multi-grade oils are modified with additives to make their viscosity less temperature dependent. This is important when the engine starts at low temperatures. If the oil is too viscous, it takes too long to circulate properly in the engine which allows metal-on-metal contact between moving parts. Also, the oil can be so "sticky" that it puts a far greater load on the battery and starter motor.

However, if the oil is insufficiently viscous, it won't lubricate as well at normal temperatures, and it could be quite inadequate during high power output, particularly on the cylinder walls which can get very hot.

The third factor is power consumption. More viscous oils may lubricate better, but they also create drag between the surfaces, and a certain amount of power is consumed in overcoming the drag. With fuel prices constantly on the increase, the trend is to reduce the viscosity to improve fuel consumption. My truck runs on 5W-20, which is pretty runny stuff!

In "ye olden days", prior to the availability of multigrade oils, it was necessary to use different oil grades in summer and winter. That's generally no longer necessary except in locations that have very extreme variations between winter and summer temperatures.
 

Offline graham.d

  • Neilep Level Member
  • ******
  • Posts: 2208
    • View Profile
What oil viscosity is best for a car engine?
« Reply #5 on: 25/05/2011 09:55:07 »
I'm not sure that viscosity and lubrication ability are synonomous features, Geezer. The synthetic oils do cover all the bases in this regard I think. The downsides are:
1. They are expensive
2. The low viscosity means they find any leaks
3. I guess that this means they can slip past the piston rings in a worn engine too
4. The composition can act as a solvent on certain plastics not designed to use them, so care is needed with older engines
5. Engines used to need "running in". Older oils allowed the lack of lubrication they provided to permit this wanted wear to occur. The synthetics do not. Fortunately modern machining has improved so as to remove (or at least reduce) the need for this.
 

Offline Geezer

  • Neilep Level Member
  • ******
  • Posts: 8328
  • "Vive la résistance!"
    • View Profile
What oil viscosity is best for a car engine?
« Reply #6 on: 25/05/2011 17:06:34 »
Well, yes Graham. It is a slippery subject. But unless you are into F1 racing, you might be a lot better sticking with a multigrade, or even a single grade mineral oil, particularly if your engine is air cooled. I actually had a very bad experience with a synthetic when they first appeared, so I've tended to stay away from them since.

I suspect synthetics have as much to do with lubricating peoples wallets as anything else  ;D
 

Offline SeanB

  • Neilep Level Member
  • ******
  • Posts: 1118
  • Thanked: 3 times
    • View Profile
What oil viscosity is best for a car engine?
« Reply #7 on: 25/05/2011 20:21:30 »
Tribology is the science of oils and lubrication, or more specifically friction. The best is to use what the manufacturer of the car recommends , as they generally choose the oil in the design of the engine. Using an oil that is too heavy will result in extra loss in the engine, and will result in higher wear as the oil film tends to become too thin due to insufficient flow. Too low an oil viscosity will also be a problem, as the thinner oil, while it has less loss, also has a thinner film due to more being pressed out of the bearing surfaces.

Generally you can use either the correct oil or the nearest step up or down, ie if you need a 15W40 you can use a 10W40 or a20W40, or a 15W30 or a 15W50, provided that you use a higher number in warm climates and a lower number in cold climates. But this is a general rule, and in all cases it is at own risk, and YMMV, and really only to be used in case you cannot get the correct oil.
 

Offline CliffordK

  • Neilep Level Member
  • ******
  • Posts: 6321
  • Thanked: 3 times
  • Site Moderator
    • View Profile
What oil viscosity is best for a car engine?
« Reply #8 on: 25/05/2011 22:59:36 »
One thing to keep in mind.
If you have a "Classic" vehicle.

Some of the websites have a discussion about what they call "Flat Tappets", and that many of the newer oils don't effectively lubricate the older tappets.  Air cooled engines may also depend on the heat conduction of the oil which may not be the same with all oils.
 

Offline Geezer

  • Neilep Level Member
  • ******
  • Posts: 8328
  • "Vive la résistance!"
    • View Profile
What oil viscosity is best for a car engine?
« Reply #9 on: 26/05/2011 06:04:49 »
Air cooled engines may also depend on the heat conduction of the oil which may not be the same with all oils.

It's not so much the variation in heat capacity; it's because the additives in some multigrade oils break down at high temperatures and permanently alter the properties of the oil. This is a greater problem in air cooled engines because they are actually "oil cooled" to some extent.

Even water cooled engines depend on the oil to cool them. If I remember correctly, oil is sprayed inside the pistons to cool their upper surface.

 

Offline Geezer

  • Neilep Level Member
  • ******
  • Posts: 8328
  • "Vive la résistance!"
    • View Profile
What oil viscosity is best for a car engine?
« Reply #10 on: 26/05/2011 07:57:30 »
Taking off on a slight tangent, does anyone think automobile manufacturers deliberately discourage owners from changing the oil and oil filters on their vehicles?  Surely it would not be difficult to greatly simplify the oil removal process and mount the filter in an easily accessible location?

I don't suppose they could be making it just a bit more difficult than it really needs to be for commercial reasons?

(I got rid of my Volvo XC when I figured out the #$%^&s were charging me $20 for a stinking paper filter that was worth about about fifty cents - not to mention the special Volvo tool they wanted me to buy to remove the expletive thing.)
 

Offline graham.d

  • Neilep Level Member
  • ******
  • Posts: 2208
    • View Profile
What oil viscosity is best for a car engine?
« Reply #11 on: 26/05/2011 10:22:57 »
Taking off on a slight tangent, does anyone think automobile manufacturers deliberately discourage owners from changing the oil and oil filters on their vehicles?  Surely it would not be difficult to greatly simplify the oil removal process and mount the filter in an easily accessible location?

I don't suppose they could be making it just a bit more difficult than it really needs to be for commercial reasons?

(I got rid of my Volvo XC when I figured out the #$%^&s were charging me $20 for a stinking paper filter that was worth about about fifty cents - not to mention the special Volvo tool they wanted me to buy to remove the expletive thing.)

Few cars today are designed for DIY maintenance. It is very annoying. Even changing a lightbulb can be a difficult job with ridiculously restricted access; and changing the timing chain/belt on many cars can mean having to lift out the engine.

You can certainly think that maybe there is some mutual backscratching going on between the manufacturers and their dealers (often incorporating a lucrative service business). On the other hand you can also see that they may feel that they get less problems, and a better reputation, if all the servicing is done by trained staff with all the right equipment (rather than the large hammer and approximate sized spanners I used to use).

To be fair, modern cars are hugely better in every respect than the dreadful mass-produced rust buckets of the 1960s. Now filled with electronics: sophisticated engine management, self-diagnostic systems, hi-fi media, GPS etc. there is so much more than can go wrong but, generally, reliability has improved a lot. Vehicles have improved also in their speed, safety, efficiency, comfort and also, I suspect, in the overall average maintenance costs in real terms if carried out by the dealers (usually only once per year or 12,000 miles).

It's still annoying though.
 

Offline CliffordK

  • Neilep Level Member
  • ******
  • Posts: 6321
  • Thanked: 3 times
  • Site Moderator
    • View Profile
What oil viscosity is best for a car engine?
« Reply #12 on: 26/05/2011 11:15:51 »
(I got rid of my Volvo XC when I figured out the #$%^&s were charging me $20 for a stinking paper filter that was worth about about fifty cents - not to mention the special Volvo tool they wanted me to buy to remove the expletive thing.)
You could always try my FIAT.
You simply clean the filter and put it back on.   Of course it is a pain in the #*&()*&)(*&(*^%^*&^%&^$^!!!!  to get off....  but there is nothing to buy but the oil.  And no..  it doesn't have a timing belt either.

My thought about the new cars is that the engineers simply don't care about the inevitable maintenance. 
With the new sensors and diagnostics and computer interfaces.  It would be trivial to put a manual into the computer.  So...  if the computer thought that it had a faulty reading from the mass air flow sensor, rather than flashing a "check engine" light, it could name the part, give a photo of the part and a diagram of where to put it in the engine compartment.  A lot of people would still take their car to the dealer, but it would at least keep the mechanics honest.

Why should replacing a $20 clutch be a $1000 repair?  Obviously it is a pain to get to.

And if a timing belt actually needs to be replaced every 80,000 miles, it should be able to be slipped off without removing any other parts.

Technically a used car should last forever.  I think the manufacturers count on them becoming too expensive and too much of a pain to maintain.
 

Offline graham.d

  • Neilep Level Member
  • ******
  • Posts: 2208
    • View Profile
What oil viscosity is best for a car engine?
« Reply #13 on: 26/05/2011 14:27:09 »
Good points. The manufacturers will certainly have a "life" figure in mind. I still think this is much higher than in the past though, so there is progress. I do regret the throwaway society we live in today; you could tell this if you ever saw my garage (no room for a car).

To be fair, I do prefer a Mercedes to a Citroen 2CV though.

BTW, you can get hold of the diagnostic software (for a PC) that is used for BMWs or for Mercedes (for example), I have been told. I don't know whether this is "official" or not. It is the same as the workshops use. I expect Haynes workshop manuals would be available too. There is the threat of invalidating guarantees to consider though.
 

The Naked Scientists Forum

What oil viscosity is best for a car engine?
« Reply #13 on: 26/05/2011 14:27:09 »

 

SMF 2.0.10 | SMF © 2015, Simple Machines
SMFAds for Free Forums