How to Determine the Correct Oil Grade for your Car

AEHaas

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Guess Ford Coyote can take it and Ferraris can't ... :D Of course the best practice for any vehicle is to not hammer the RPM until the after the oil is warmed up to near full operating temperature.

Maybe the oil is too thick when the engine is not fully warmed up and the thicker oil cannot lubricate properly? Hence the development of multi viscosity oils?
 
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WA
I think development of multi viscosity oil is another story/subject. You still have to wait for optimal operating temperatures regardless.
 
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43,676
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'Stralia
A safety margin is not one ended. Too thick an oil, especially at higher RPM, may result in the worst type of wear within moving parts, cavitation. This is when chunks of metal are pulled off of the surfaces. If the oil is a little thin and the moving surfaces get close then the additive package comes into play to prevent or minimize any wear. Nothing can prevent cavitation if the fluid cannot keep up with the movement of parts.

AEHaas
Cavitation ?

Where specifically is that an issue ?
 
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'Stralia
Yes, the flow would be the same until the relief pressure was hit. The pressure would be less (with thinner oils) but the flow would be the same. I think it is time to revisit my original articles of 16 or 17 years ago. When the oil is at my Florida start up temperature the pop off valve is already hit. The pressure at idle is 80 PSI. That means that at any RPM from then on, before the oil temperature gets up, the flow is the same. There is no increase in flow with increasing RPM. 'Not good in my book.
Flow (in the bearing) is determined by the viscosity, bearing diameter, bearing axial length, clearance, and rotational speed.

The flow through the bearings certainly DOES change with RPM for any given oil pressure...the bearing takes what it needs, the rest builds gallery (back)pressure and flows out the relief.

More info here....

 
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WA
Cavitation ?

Where specifically is that an issue ?

in general and big picture I still don't understand where op is coming from? Otherwise, I enjoy reading technical discussions even though I am not an expert. The problem is many inconsistencies!
op contradicts himself sometimes within the same post and/or different threads.

Regarding engine oil viscosity:
For example in another thread he was basically saying that the manufacturers know what they are doing ... and if it was that easy to design engines we would all be making them ...
If that's the case, this discussion is irrelevant. Refer to the title: "How to determine correct oil grade for your car".
Just follow the manufacturer's recommendation!

In this or another thread he is using 0W-20 and the factory recommendation is iirc 0W-40 (town) and 10W-60 (racing) ...

at least I can understand those who use heavier than recommended oil viscosity. The logic (flawed or not) is that they don't trust the manufacturer due to CAFE. but the reverse (going much thinner) doesn't seem as logical.

Basically go by spec if you fully trust the manufacturer. Otherwise (CAFE theory) you have to go thicker.
Again, flawed or not those are the only two logical options.
I don't understand going thinner! Please help. lol
I am open to change my mind.
 
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35,970
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Basically go by spec if you fully trust the manufacturer. Otherwise (CAFE theory) you have to go thicker.
Again, flawed or not those are the only two logical options.
I don't understand going thinner! Please help. lol
I am open to change my mind.
I like this statement. And "I and don't understand going thinner" either. Here's my spin and the spin of several others on this site regarding trusting the mfg. and CAFE. There are parts of the world where the mfg. specs "thicker oil" than they do in the US because they don't have to deal with CAFE, in the same exact engines we have here. That told me something long ago, and for my vehicles it's still true today. I'd rather give up the fractional fuel savings that I can't measure for the added protection respected members and experts are telling me I will get by bumping up to a 30 grade from a 20 grade oil. I sleep better at night with a xw30 oil vs. an xw20, and at the end of the day that's all that matters to me. This horse will be beaten to death over and over again for years to come. ;) Bottom line do your homework and use what makes you sleep better. Trying to change someone else's mind imo is a waste of time. Sharing opinions and data if available can help someone make their own choice.
 
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I'd rather give up the fractional fuel savings that I can't measure for the added protection respected members and experts are telling me I will get by bumping up to a 30 grade from a 20 grade oil. I sleep better at night with a xw30 oil vs. an xw20, and at the end of the day that's all that matters to me.


Last sentence in the linked article/PDF:
"Hence, in general, you are always safe to go one grade higher than the one recommended by your engine manufacturer, but never use thinner oils than recommended."
 
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2,016
Last sentence in the linked article/PDF:
"Hence, in general, you are always safe to go one grade higher than the one recommended by your engine manufacturer, but never use thinner oils than recommended."
This right here.

First off, I recommend the oil viscosity that is printed in the owners manual, I’ve stated that countless times throughout the years. I’m a fan of thinner viscosity oil if the engine allows for it.

Second off, I believe stepping up one viscosity grade is fine. If you plan on towing or running your engine harder than normal, stepping up a viscosity grade is fine, and often recommended in many OM’s.

I tend to stay with the grade listed in the OM, but it would seem safe to go up one viscosity grade if needed/desired.
 
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Italian cars are finicky about oil choices. My baby Ferrari 128 SemiFast would run fine on 10W-30 at any rpm. I wouldn't trust larger displacement models with that viscosity.

View attachment 51293
Ah, the FIAT 128SL. I enjoyed one back in the late 70's. Found a nice trade at a Ford dealer. Previous owner got a tune-up and a plug wire was off - the engine was running on 3 cyls. I Heard that, looked under the hood and popped the wire back on. Viola!
Great car fun to drive. I think I paid $1700 - it was a low mileage car. Inspiring Ferrari designed engine.
A bit hard to get to the distributor, the thing was right up against the radiator, IIRC. I Know this as I was getting misfire at higher rpms.
When the dist advanced it was shorting the points spring against the dist inner housing. Not the finest Magneti Marelli design for sure.
I got it all fixed up. Repaired someone's previous, ham fisted "Tune DOWN" !

- Ken
 
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I enjoyed that 128sl. It had a very nice 1300cc engine. Mine had a Webber two barrel carb. It was a very tossable 🇮🇹 car. Front wheel drivers were uncommon back then, 1974. Wish I still had it. The tin worm ended its life.

Italian cars are supremely satisfying to drive and equally frustrating to maintain. Certainly not for the faint of heart owner.
 
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