Blimey - this is one huge thread and would take all day to read. So maybe it's time to go back to one basic thought.

For the benefit of any simple souls out there with a standard WRX Bugeye and no wish to redline (more than only occasionally) or do track days etc., all you need to know -as recommended by OILMAN is that Silkolene ProS 5W40 is a good all round oil.

 

Tony, I'm familiar with the 300V oils, used both the 10W-40 and 15W-50 when I owned an Impreza.

Subaru don't specify 20W-60 Le Mans oil, even the rally team use 5W-40.

Running an engine at too high a temperature puts it,and the oil, under much higher stresses. Whilst I am happy to agree that a higher viscosity oil may offer a degree of additional protection it isn't the full answer to the problem. Is it not better to run the engine at a lower temperature through additional oil cooling and/or having a larger oil capacity?

 

Andrew i take your point, however, i have always had a lot of involvement in grpN and production type competition cars where you are specifically not allowed to alter any of the main engine components or add coolers..............................

the 10w60 debate has been voiced time and again, but all we ever see is the fact that at a higher temp the oil would be thinner???? than another oil at a lower temp.........

what i was hoping for was some clarification as to why it would be worse to have better protection if both oils were subjected to the same temps...............in our experience, we found that higher viscosity oils improved reliability and decresed engine/component wear over other more "preferred' brands/oils!!
this is 'real world' experience and is also relevant to a lot of our road type customers who appear to give their cars a much harder time than most guys competing!!

alyn

 

Surely the answer to that is if temps don't rise high enough for the 10/60 grade to afford any advantage over a 10/50 say....it's like overcooling any oil...it needs to operate within it's optimum range for best circulation etc...which is what I've already said...

 

Sorry, I've not been recieving notification of replies on this for some reason.

Some very fair and valid points here but I could not put the answer more eloquently than "crusher" other than to say why drag an oil around the engine that is thicker than required as it has more downsides than benefits.

A shear stable, light viscosity, proper synthetic is more than capable of coping with the demands of a tuned modern engine.

In most post 2000 scoobies it's also whats recommended in the handbook.

Cheers
Simon

 

Another critical factor that you never mention it Oilman is TBN, it's more or less an index how the oil performs after few kms, wear etc.
I wonder though why Motul Chrono 10W/40 has lower HTHS number from Motul Power 5W/40 (which has wider viscosity) while Chrono has higher TBN?
JIM

 

Alyn,


In general a thin synthetic will flow and cool better and shear less than a thick mineral and therefore at elevated oil temperatures its not a level playing field.

 

not talking poor quality mineral here (or i'm not anyhow) - we are talking an ester based proper synthetic 10w60..........................
care to shed any light on this??

 

I'd only use the 60 rated oils if high temps with dropping pressure are an issue...it's all been said surely....

 

The message I'm getting seems closest to Crusher's, I think.

There's always a lot of talk about viscosity, but what I'm hearing is that it is shear strength (HTHS) which is the most important consideration in engine protection? That's the thing, if I've inderstood this correctly, which keeps the metal bits sepatated under load?

Viscosity obviously plays a part in this, which is influenced by temperature and also by the rate at which oils deteriorate with use, and oil pressure etc. But all these things apart, am I right in saying that the oil which delivers the highest shear strength most of the time will give the best all-round protection, regardless of any other factors, eg viscosity?

If so, then we have to set some parameters in terms of 'other factors' such as temperature/pressure/mileage/etc to find the oil that gives the best shear performance under the most relevant conditions, and if necessary, modify our driving to suit.

This leads me to Motul 10w-40 which seems to have enough top-end performance to withstand track days and the turbo heat - main priority. And it has a relatively narrow viscosity range meaning minimal use of volatile Viscosity Improvers which should keep it working better for longer. (I also like the idea of its inherantly sticky film properties helping to lube the cold bits overnight and when changing the oil.)

The only downside is that you must warm it up carefully to maintain cold protection, and it might be a bit gooey at first which will hit fuel consumprion and power by, oh, at least 3bhp.

Another important consideration is that my car is full of it now, and I have another 5l in the garage

Richard.

 

Indeed, the salient points are as follows:

Wide viscosity oils need more VI Improvers and are prone to shear in service more quickly than narrow viscosity oils requiring less.

Synthetics (pao/ester) are more shear stable than petroleum/hydrocracked basestocks and therefore require less VI Improvers, meaning they can stay in grade longer.

The rate of shear in service is important comparable by the HTHS numbers.

Lower viscosity oils are better at cooling (dissapating heat).

There are no downsides to using lower viscosity oils so long as adequate oil pressure can be maintained.

With regards to protection, shear stability is more important than viscosity as a 10w-40 semi-synthetic will not remain in grade as long as a 10w-40, 5w-40 or 0w-40 high HTHS (shear stable) synthetic.

The quality of VI Improvers vary and generally the best ones are used in the best oils once again HTHS numbers indicate quality used.

Hope this helps
Simon

 

Blimey, I was right then

So it's the shear strength HTHS performance that matters, and all the numbers that are commonly quoted like viscosity, oil temperature and pressure are merely contributing factors and not defining characteristics.

Also, how much work the oil has already done in a hard-driven turbo motor is a major factor, and in turn this is significantly influenced by the chemical make up which we can only guess at from reading the tin.

No wonder there is so much debate...

Richard.

 

Question: what are the main factors that degrade an oil's shear strength? Combustion contamination, how much shock loading it's been taking, being cooked in the turbo? What happens to the oil during heavy use that makes it necessary to change frequently?

And how important is oil pressure, per se? Is it not just an indicator of flow, which is the thing that keeps fresh, cool oil doing the business? If so, is it possible that an oil showing lower oil pressure figures can actually be giving more protection because of its other performance characteristics?

Sorry for any additional confusion

Richard.

 

Fuel dilution thins, Soot thickens and extra heat lowers viscosity so there is a self-correcting behaviour.

Chemical make up , esters virtually indestructible, anti wear and friction modifiers and effects on elastohydrodymanic lubrication, boundary layer and thin film, effected by liquid flow, physical chemsitry and the reaction of the oil and the surface.

 

Fuel dilution, cooking in the turbo and "good"-stable oil pressure are the keywords for me!
I have similar oil consumption with V300 10W/40 and RS10W/60 (high ambients in Greece) which means at least fuel dillution is more or less the same while pressure isn't that different between the two oils!
Which leds me to the V300 10W/40 for the summer and 5W/40 for the winter (under 10C ambients).
Of course worth to mention my car with the Motul - until it reaches working temps - works like diesel while the Castrol gives smooth and quiet warm up period.
I have an oil analysis from various Subarus in Denmark where 10W/50 oils give the best results from all other viscosity groups in different conditions/milles/cars while 10W/40 group is second.
Unfortunately there isn't even one car with V300 just for comparison.
JIM