Going down a grade is bad but shearing is not

[KL31]

I don't understand this logic. Car is specd for 30 but shears well into 20 but people with uoa don't think it's an issue. Seeing it plenty on turbo or turbo+DI cars.

Then if you ask about going down a grade from 30 to 20 for a 30 specd car people will advise to play it safe and use only what is specced...

Also what about racing? If you track your car it could see oil temps 120C or above. That would mean the oil is then well into a 20 grade if it started at 30 but most engines don't blow on the track... well they do more than everyday driving that's for sure.

Makes sense to me that some may use a 60 grade race oil on the track as the temps could shoot up and the 60 would therefore be more like a 30 or 40 at high temps.

I dunno, does it really make any difference? Can engines specd for 30 probably run 20 30 40 50 and not experience any issues, probably yes.

 

[SR5]

Years ago some Castrol guy was telling me R / Edge / TWS 10W-60 was a sixty grade to combat the massive fuel dilution experienced on the race track.

They didn't expect the oil to stay 60 the whole race, but stay reasonable the whole race. This was from memory and years ago, so happy to be corrected.

 

[Shannow]

Started a post, but...meh...

 

[SR5]

Originally Posted By: Shannow
Started a post, but...meh...

Laugh !!

How about HTHS drops at about half the rate as kinematic viscosity. As a rule of thumb. You taught me that one.

 

[Merkava_4]

I'll answer the question: Going down a grade is bad because it gives you no room for shearing like the thicker grade does. 5W-30 can shear down to a 20 grade, but what's 5W-20 gonna do ?

 

[KL31]

Originally Posted By: Shannow
Started a post, but...meh...


Well it is Friday arvo

 

[SR5]

Originally Posted By: KL31
Originally Posted By: Shannow
Started a post, but...meh...


Well it is Friday arvo

'zackly

 

[CharBaby]

Originally Posted By: Merkava_4
I'll answer the question: Going down a grade is bad because it gives you no room for shearing like the thicker grade does. 5W-30 can shear down to a 20 grade, but what's 5W-20 gonna do ?


True! However from my understanding(and I may have this wrong), a 5W20(or 0W20) compared to a 30, is more shear stable by design(HTHS) & its lower use of VIIs...for the everyday use of a vehicle over a normal OCI. I can't define normal but, what ever is discussed here at BITOG; 5K-7.5K mi OCIs? I'm sure there is a lot more to it than this!

 

[Olas]

Or use redline, it doesn't shear.

 

[CharBaby]

Originally Posted By: Olas
Or use redline, it doesn't shear.

All oil shears however, Redline may not shear out of grade as much as others, IDK!

 

[Rolla07]

Again its splitting hairs because 99.9% of ppl wont see a difference in using 5w20 vs say even a 10w60 in terms of engine durability/longevity regardless of all our analysis. Thats why I would choose an oil based on use and conditions. My car can run 5w20 or 5w30, others can too..

If car is tracked id use race oil 10w60 or 15w50.
If I had a DI vehicle I would use mfr recommended while in warranty but keep the oil changes to shorter intervals or follow the severe service.

Mfr recommendations take into account what most people do with their cars (normal everyday driving)

 

[09_GXP]

Shearing is not this magical phenomenon that only BIOTGers know about. Car companies spec 30wt and will fully expect it to shear to 20wt under certain conditions. The owner's manual only provides a spec for new oil, NOT used oil. Going with a lower weight new oil violates the spec, hence concerns over warranty.

 

[ARCOgraphite]

Oils don't "change grade" when heated - this is just the function of KV T.

Many "serious" long duration competition cars utilizing a dry sump will heat a high viscosity/ high HTHS oil MONOGRADE prior to starting the engine (and the race) then cool the oil as needed to keep it in the viscosity range require for the event duration.

 

[BrocLuno]

Originally Posted By: Merkava_4
I'll answer the question: Going down a grade is bad because it gives you no room for shearing like the thicker grade does. 5W-30 can shear down to a 20 grade, but what's 5W-20 gonna do ?


+1

But ... It depends on the oil in question. And how do you define "lot more stable" and which oil are you talking about ...

 

[4WD]

When we had TBI and then sequential port injections - this did not bother me as much. But with GDI and turbo. - there are good reasons they went back up to 30 ... and once you pile on the miles - one can ponder a "35" as BL mentions sometimes ...

 

[tig1]

Originally Posted By: Merkava_4
I'll answer the question: Going down a grade is bad because it gives you no room for shearing like the thicker grade does. 5W-30 can shear down to a 20 grade, but what's 5W-20 gonna do ?


I used M1 5-20 in the late seventies in engines calling for 10-40. No problems.

 

[Virtus_Probi]

Originally Posted By: KL31
Originally Posted By: Shannow
Started a post, but...meh...


Well it is Friday arvo

Isn't it already Monday down there??
Something about the equator and the international date line intersecting...


;^)

 

[Nick1994]

Because it's probably a 30 grade for most of the interval, and then slowly slips into a 20 grade toward the end.

Rather than a 20 grade 100% of the time.

 

[Errtt]

My DD spec 5w-20. I use 5w-30. Maybe it shears close to 20 maintaining close to what the spec is most of the OC duration. Maybe they wanted 20 for some CAFE reasons. I dunno. About 117k and it still runs fine and no oil added during the OC duration.

 

[BrocLuno]

Yeah, but the OP is talking about a Direct Injected engine, and that's a fuel dilute'r for sure. So not only do we have mechanical shearing, but fuel dilution. How long that 30 stays even close to MFG grade depends on the engine topology and the amount of fuel getting by the rings ...

The classic wisdom is that oils tend to volatilize in the ring band. That means they are going up in viscosity as they give up some of their components. It also means that they can fight friction better because they can maintain metal to metal separation. That's why we still see cross hatching on the cylinder walls in modern motors at 100,000 miles.

But, with GDI, we are seeing these thicker fluids washed off the upper walls (part of the fuel dilution process on the way to the crankcase). So now you are depending on just a 30 to keep the ring pac and upper walls happy, and that's being both sheared down (sliding motion) and washed away

If I owned a GDI, I'd be running the stoutest 5W-30 I could get my hands on, if not a 5W-40 for peace of mind. And I'd be short changinging the oil - prolly 5K max. Not looking to go down a grade for some reason... That's backwards to what's happening in the engine.