Ambient Temperature vs. Viscosity Choice

[Glenda W.]

My 2018 Outback has an oil temp reading in the infotainment unit. I can say that my oil temperature does get impacted by ambient air temperature. I normally run 200-210. When temps get over 90 the oil temp can go into the 230’s.

 

[GRUMPYDUDE2]

My 2018 Outback has an oil temp reading in the infotainment unit. I can say that my oil temperature does get impacted by ambient air temperature. I normally run 200-210. When temps get over 90 the oil temp can go into the 230’s.

. Agree.
I see this in every car or truck I have ever owned.

 

[kschachn]

I wasn't sure; doesn't convection have to do with the actual convective motion of the fluid/gas? As in, it heats up, rises away from the heat source, and then cools and sinks?

Either way it's not radiation, but the action of the air over the pan that transfers most of the heat. And oil pans aren't really designed to transfer much heat.

This is forced convection.

And radiative heat transfer is always a factor given the large about of transfer that can occur. But as I noted I think it’s mostly convective as long as the vehicle is in motion.

 

[oil pan 4]

I if you know peak oil operating temperature chose the second number accordingly. You're looking for about 10cSt viscosity at operating temperature. Nearly every single gasoline powered, liquid cooling vehicle engine is going to run about 200f to 230f oil temperature.
Super hot for oil is like 300f.

 

[CarlB]

I don't see that in my experience. On a 30 degree day, my water temps are around 203, and on a 105 degree day, they're exactly the same.


It's not an all-open/all-closed valve that just opens up at the set temp. Instead, it's a valve that's controlled by something called a 'wax thermostatic element', meaning that there's a container of a specific sort of wax in there with a piston, and that wax expands and contracts a specific way at specific temperatures.

So when the car's cold, that wax is solid, and the valve's closed. As the car heats up, it expands, opening the valve accordingly with the temp. And it does this continuously as the car is in operation. The whole thing is engineered to keep the temperature at the set-point, say 203. So if the incoming cooling water is above that, it opens up a little bit more because the wax expands a little bit more. If it is below that, it shrinks the wax a little, restricting the flow a little and bringing it back up to temp.

And I seriously doubt automakers engineer their cooling systems such that they're wide open at the operating temp. That's just idiotic- far better to give some cushion in there by setting the thermostat up such that it's got some wiggle room to open up further than that.

That's why I don't see the temps fluctuate- that thermostat's doing its job and keeping the water temp where it's supposed to, independent of outside temp.

That's fine in your situation, but not all setups and behavior will have identical temperature in any climate.

 

[CarlB]

What I'm getting at is that 20 or even 50 degrees Fahrenheit isn't going to affect your oil temp that much if your cooling system is functioning properly. Yes, the oil temp may vary a little b

I wasn't sure; doesn't convection have to do with the actual convective motion of the fluid/gas? As in, it heats up, rises away from the heat source, and then cools and sinks?

Either way it's not radiation, but the action of the air over the pan that transfers most of the heat. And oil pans aren't really designed to transfer much heat.

That's making a lot of assumptions though. Light usage vs heavy usage might change that.

Which is why I mentioned data. If the oil temps and pressure are changing, then you may adapt. If you are already hitting perfect temperature numbers, then there's nothing for you to worry about.

 

[ATex7239]

It can’t be just run the recommended viscosity and call it a day. No, that’s too easy. Surely that cannot be enough.

You gotta run this viscosity in this weather. And that viscosity in that weather. And if you move across town the elevation goes up 100ft so you have to compensate for it. Don’t forget during the holidays you gained a couple lbs so you have to adjust for the increased load.

People like to feel like they’re doing something “better.”

 

[Jetronic]

Having been around here for nearly two decades, I've read more comments talking about how people run higher viscosities when it's hotter outside, and vice-versa. This makes a sort of intuitive sense, as oils, even multigrade ones, thin out as temperatures increase. And that phenomenon explains why we'd want lower "W" numbers on multigrades as well- that indicates thinner oils at start-up temps, which are considerably cooler than within a warmed up engine.

But the part I can't quite square is the thinking that somehow a engine that specifies using 30 weight (or 20, 40, etc...) and has a say... 203 degree thermostat is going to somehow require heavier oil when the ambient temp is high outside.

The reason this doesn't really make sense to me is because the difference between "comfortable" and "super-hot" is like 25-35 degrees Fahrenheit, and is *still* 100 degrees F below the operating temp of the vehicle. The assumption is that the cooling system is so feeble that driving it in say... 105F temps is going to overwhelm the cooling system and necessitate higher viscosity to offset those temps. This would be the same if we're talking bearings, cams, etc.... the cooling system would have to be nearly ineffective for the ambient temp to really matter, provided the oil is in the right spec, as it's trying to keep everything right around 203, and the *internal* heat generated by combustion, in the bearings, etc... is what needs to be shed. The only place ambient temp comes into it would be in being able to shed the heat that the cooling system is moving from inside the engine to outside, and that's affected by the ambient temp. Even then, the main thing is going to be the internal heat generation rate, not so much the ambient temp, provided it's not something absurd like 120F outside.

What am I missing, if anything?

The cooling system "regulates" the coolant temp, not the oil temp. The coolant receives most of it's heat from the combustion events, the oil system more from shear, and is generally cooled in/by the sump The sump sees ambient air at best, or under hood heated air at worst. Turbo engines usually have an oil cooler but that is sized so small that it becomes pretty unreliable when the engine is run hard. They are only effective when there's a big enough difference between oil and coolant temp.

If you can actually monitor oil temps, a thicker oil might not be needed, but be aware that a LOT of cars don't even have an oil temp sensor, and the oil temp reported on the CAN is pure fiction.

 

[mark pruett]

. Agree.
I see this in every car or truck I have ever owned.

Fair enough, but does that even matter?

That's kind of my point- we've got people saying to use 40 weight oils when the oil temp increase due to ambient is *maybe* 20 degrees higher, and still well under any sort of performance issue threshold.

At that rate, the ambient temp would have to be like 150F for it to make a difference, and even then

 

[wpod]

I use 0W-40 euro oil in every gas engine I own that Covers close to -40* to over 100* F empty towing etc.

 

[GRUMPYDUDE2]

Fair enough, but does that even matter?

That's kind of my point- we've got people saying to use 40 weight oils when the oil temp increase due to ambient is *maybe* 20 degrees higher, and still well under any sort of performance issue threshold.

At that rate, the ambient temp would have to be like 150F for it to make a difference, and even then

Most likely no.
But I get uptight when I see oil temps above 230f . Makes me feel better with 0-40.