Oil Change - Hot or Cold

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27,168
Location
PNW
it's like humidity in air, it's relative to temperature. The humidity condenses out of air when it gets cold, the precursors to varnish drop out of oil when cold. The humidity clings to cold objects, the varnish precursors to metals.

It doesn't matter if relative humidity is low, you won't get to the dew point anyway, and it doesn't matter if the contamination is low in the oil.
Guess what I'm saying is that no matter if the oil is drained hot or cold, the oil film that stays on all the drained off parts still gets cold and any varnish precursors will still drop out when it cools. There is nothing that can be done to remove the oil film off of all the parts inside the engine except tear it apart and clean all oil off with solvent. If a hot sump cools before draining the oil, are you saying the oil pan coud become more varnished compared to if it was drained hot?
 
Messages
5,371
Location
down in the park
If the hot oil gets drained, and new oil put in before the engine cools, the precursors will be absorbed by the new oil, and not drop out. Their concentration will drop significantly in the whole of the engine.

The coolest metal parts in the engine seem to varnish most. The oil pan will always have the most oil in it and can absorb more contaminants for that reason.

We've all seen the varnished and sludged engine pics, you know where the contamination prefers to go.
 
Messages
5,371
Location
down in the park

Go to periodic inspection and maintenance, then scroll down to Operations after the engine is warmed up. Item E3 is change engine oil.

Even car makers procedures say to do it after warming the engine up yet the car was on the lift prior for operations under the engine. They don't say to do it for diffs, transfer cases or manual transmissions.
 
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1,201
I prefer slightly warmed up, rather than scolding hot. I might start it and let run for a little bit, just in hopes of suspending contaminants in the oil, but I’m nit letting the thing get to operating temperature so I can burn myself under there and feel miserable about something I enjoy doing (changing oil).
 
Messages
5,371
Location
down in the park
Scalding hot oil is only a concern in summer anyway, and the temperature of the sump is a good indicator. In winter, the sumps are rarely more than luke warm, especially if the car has been sitting not running for a while, when the owner goes to the service advisor etc.. Quick lube places where the customer drives onto the pit, those could be a bit hotter, especially in summer.
 

4WD

Messages
16,628
Location
Texas
Scalding hot oil is only a concern in summer anyway, and the temperature of the sump is a good indicator. In winter, the sumps are rarely more than luke warm, especially if the car has been sitting not running for a while, when the owner goes to the service advisor etc.. Quick lube places where the customer drives onto the pit, those could be a bit hotter, especially in summer.
Exactly … I think warm oil is good enough for this on ramps (front high):

98D2AB80-FF46-4FAB-A7A7-313D554F6BE0.jpg
 
Messages
1,627
Location
California
I would never change my oil when it is truly cold, unless strange circumstances made it necessary. But also I don't drain it as soon as I shut off the engine either. And it just seems silly to start a cold engine just to warm up the oil for a drain. Look, the car gets driven, right?
I wait until it's been running at full temp for at least 15 minutes (work commute or running errands or whatever). After shutoff, I wait about 15 minutes so the exhaust will not burn me and I wear nitrile gloves so the oil won't scald me.
Thinking about hot exhausts and hot oil, I'd hate to work at a quick lube where I'm changing oil on engines at full temp, over and over, in the hot summer, while stuffed down into one of those underground service bays. Ugh.
 
Messages
5,716
Location
the canyons
On my own stuff which I care about, I always change the oil when it's at full operating temperature, and any impurities are more likely still suspended in the oil and will drain out.

As I've mentioned on this topic before, I've witnessed what can be left behind when the oil is drained cold, even if it's allowed to drain overnight.

People can do whatever they want. I'll continue to drain oil while hot.
 

e40

Messages
386
Location
Wisconsin
Hot. Soon as I pull into the garage after work, in goes the MityVac 7201 evacuator. I'll pump it a couple dozen times, and go inside and change clothes, by the time I get back I can usually take off the filter, and pump more out with the MityVac. Been doing it that way for years, had a Pella before the Mity.
 
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