Why does the high ambient temperature matter?

[Bdog]

Since engines are liquid cooled and have a set operating temperature why does the upper ambient temperature matter when selecting an oil grade? I totally get the low ambient temp being important because the oil has to be able to flow at startup in a cold engine but the engine gets to 200+ degrees at operating temp. Why for example on one chart will they say a 5w30 good to 104 degrees and a 5w40 is good to 122 degrees?

Where I live I have seen up to 117 so it is relevant to me but I just can't grasp what difference it would make because the engine gets so much hotter than the air temp and it stays constant for the most part. My engine reads around 205 degrees regardless if it is snowing outside or the middle of summer.

 

[Shannow]

Not sure…the oil in the sump of my Caprice (3.8L supercharged V6) will get to 110C whether it’s freezing or 30C after 20 mins highway.

 

[zach1900]

You're right, I don't understand why people think a water cooled IC engine has a wildly different operating temp. More like 1 or 2 degrees.

 

[MNgopher]

I can read the oil temp in my two vehicles through a scanguage. You are correct that in general, the water temp doesn't fluctuate all that much no matter the outside temp.

The oil temp on the other hand can vary dramatically. On my F150, the water temp can be a consistent 191-193 ish, regardless of the outside temp. The oil temp on a zero degree day will struggle to hit 170 (remember the water temp is still 191 ish). On a 100 degree day, the oil temp will be over 200 degrees (with the water temp still 191-193 ish).

 

[mjoekingz28]

If you need set proof, just change the oil in winter then again in summer. In the winter, you can take an hour trip and immediately change the oil and probably notburn yourself, do this in mid-July and you'll get the point.

However, I dont understand why some manufacturers say 5w30 for Any climate, while others say 10w30 between, say, 10-90f, 15w40 from 32-100 so on and so forth.

 

[AKhan87]

Originally Posted By: MNgopher
I can read the oil temp in my two vehicles through a scanguage. You are correct that in general, the water temp doesn't fluctuate all that much no matter the outside temp.

The oil temp on the other hand can vary dramatically. On my F150, the water temp can be a consistent 191-193 ish, regardless of the outside temp. The oil temp on a zero degree day will struggle to hit 170 (remember the water temp is still 191 ish). On a 100 degree day, the oil temp will be over 200 degrees (with the water temp still 191-193 ish).

+1, oil temp is not the same as water temp(displayed on your temperature gauge inside vehicle).

thanks mngopher with the oil difference your provided, I didn't know it varied THAT much. A difference of 30 or more degree is considerable in oil operating temperatures.

 

[zpinch]

Yup, oil temp varies much more than coolant temp, that is why I installed an oil pan heater on my CRV with a bypass filter setup, I WANT the oil temp to get up there as quick as possible.

 

[eljefino]

There's a large difference in how a human running at 98 degrees perceives 80 vs 100 outside. A car running at 198 just needs 20% more reserve capacity.

We project ourselves onto our machines, and figure they'll wilt and need extra "protection", and that the car's engineers couldn't fathom the extreme weather we live under.

 

[zach1900]

Couldn't have said it better.

 

[zach1900]

In some cars is not that much, that was his point.

 

[WobblyElvis]

Pulling a load up a hill at 110F requires a lot of coolant to pass through the rad and the thermostat opens all the way. Light load in winter and the thermostat opens just a crack. If you read the specs on a thermostat you will find a 15-10 degree difference between these two conditions.

Of all the heat lost by the motor about a %30 is just from surface cooling. This would mean the oil temp in winter is much lower.

 

[440Magnum]

Originally Posted By: Bdog
Since engines are liquid cooled and have a set operating temperature why does the upper ambient temperature matter when selecting an oil grade?


Because even though the coolant temperature may be thermostatically controlled to within the setpoint +10-15 degrees F, the oil temperature of most engines is unregulated. It falls where it falls, and depends fairly strongly on ambient temperature of the air flowing over the oil pan or oil cooler.

The engines that have oil-to-coolant type oil coolers (like my SRT) are more immune to ambient temp, but they still have quite a bit more variation of oil temperature than coolant temperature, and it depends significantly on ambient temp. I can see that on my oil temperature display as the seasons change. Oil temp on the SRT rarely tops 210 in the winter, but 225-230 in the summer is about the norm.

 

[geeman789]

And that 20*f difference in oil temp equates to a grade difference in viscosity...

Your 20 grade in the winter is now a... ?? grade (16...?) in the summer...

Never really understood why some are so against a slightly thicker oil in the summer... that used to be standard for every car, until CAFE roared...