Why is it OK to put cold oil into a hot engine?

[Fenixgoon]

crazy question for you all. My neighbot drains the oil, then cranks it over for a few seconds. Claims he gets another ~1/2 quart of dirty oil out that way. Is he crazy, or is this common?

i wouldn't trust myself to not seize the engine

 

[yottabit]

The temperature difference or thermal "shock" to the metals will be even less when you take into account the temperature difference between the hot and cold oil is not equivalent to the surface temperature of the motor

There is a heat transfer coefficient that will determine the rate at which the oil can conduct heat to (or away from) the motor internals, and then that only determines the temperature at the actual surface of the metal, heat is then conducted through the metal and the temperature difference decreases even more

If you spray down a red hot turbo housing with enough ice water then you would probably run into damaging thermal shock, but both the temperature differential and heat transfer coefficient would be much higher in that case, and you would be talking about a more brittle material (cast iron)

 

[Throckmorton]

http://www.bimmerfest.com/forums/showpost.php?p=5628422&postcount=1

Another couple charts that split the temps of the coolant in the engine and the coolant in the radiator along with the oil temp.

This has me thinking that 5W-20 isn't such a bad idea after all, especially for short trips. My daily commute is 5 miles each way to the bus station which takes about 15 minutes. My oil probably never reaches 100C. 5W-20 is 15% less viscous than 5W-30 at 100C, and I think the difference at 72F is even greater even though the 5W number is the same, which means it's flowing better for more of that short drive. Or maybe 0W-30 would be the best of both worlds.

 

[Pulsar]

My Brother's 2012 Grand Cherokee shows the oil temp, coolant temp, trans temp, and oil pressure on the EVIC. I think an experiment is in order.

Maybe start from cold and record each one every minute for 10 minutes?

The Oil pressure may not be true pressure. Many companies have gone to a simple on/off switch to save money.

 

[LTC8K6]

The Oil pressure may not be true pressure. Many companies have gone to a simple on/off switch to save money.

Where are the psi numbers coming from, then?

 

[pcgeek11]

You change the oil after running the engine so that more contaminates are suspended in the oil and will be flushed out when it is drained.

That is why you run the engine before changing the oil, it isn't a matter of hot and cold.

Granted hot or warm oil will flow better than cold oil, but that is not the reasoning.