Oil Change - Hot or Cold

Messages
17,759
Location
NH
Cold. Stone cold. I've changed it at 20F and didn't think twice--if anything, it was easier to roll around on the ground, all bundled up, yet the ground was nice and solid, no worries about rain etc. My current daily has the oil filter behind the exhaust manifold and it is a burn hazard IMO.

DESPITE doing 10k oil changes I just don't think my new-ish fleet somehow has the equivalent of sandpaper flowing through the oil passageways--the oil is far from done, and it's far from loaded up. And I don't think warming it up will suddenly make it vacuum up any deposits that had dropped overnight into the pan.

Now, if I was running 15W40 and needed to do an oil change at 10F, then yeah, running for a bit so as to get it thin so it'll flow in decent time, ok I'd do that. And old school differentials, with gear oil, that is best done warm, no doubt about that.

Your mileage will very likely vary here.
 
Messages
6,524
Location
Los Gatos, CA
When using the MityVac extractor, warm oil does help. I have not extracted hot oil.
When pulling the drain plug, I prefer cold. Warm is OK. Hot is too hot especially if I have to reach in to get to the oil filter.
No fun working on a hot engine either way. If it is too hot, I might pull the plug and come back later, time permitting.
 
Messages
17,759
Location
NH
That's true, when I had the VW I did topside oil changes, and yeah warm oil worked much better with that thing tubing. Got me on that one.
 
Messages
227
Location
eastern WA
So is the point of draining warm to get contaminants into suspension, or is it just to make the oil drain faster? If it's the former, does it really matter whether the oil is warm?
 
Messages
3,719
Location
Nashville, TN via Memphis
If it’s over 80°F or so outside, I don’t worry about warming it up.

Yesterday, when it was 25°F outside, yeah, I took the 4Runner out on the freeway for a 15-20 min drive to warm it up before draining.

Even then, after I rounded up everything I needed and got the skid plate off, it was just warm coming out.
 
Messages
227
Location
eastern WA
yes it matters for the first, it doesn't matter much for the second part.

Look up anniline point, the higher it is the more dropout of contaminants you get when the oil is cold


does this basically mean that running a cold engine for one minute is sufficient as long as you are willing to wait for all of the oil to drain?

fwiw, this is not an academic exercise for me. i have an old car in a cold shop, a snowy driveway, and i want to change my oil one night this week without filling my shop with fumes. so if i can run the car for a minute, shut it down, pull the drain plug, and let it drain overnight, then refill the next night, that's a lot easier than trying to warm up my engine.
 
Messages
17,759
Location
NH
I wouldn't think one minute is long enough to get much heat in.

What about after you come home? Engine is hot--so raise the hood and come back in 10 or 15 minutes? or 30, if needed.
 
Messages
27,234
Location
PNW
the varnish you see in engine teardowns is the result of stuff that gets suspended when hot and deposited when cold.
Generation of varnish isn't effected by if the oil is changed hot or cold - if that's what was meant. Even if oil is drained at 200F, the same film of oil is going to remain on the parts that might become "varnished". The oil film left over on the valve train, valve covers, etc is the same every time the engine is shut off, regardless if the oil is drained hot or cold.
 
Messages
5,379
Location
down in the park
Generation of varnish isn't effected by if the oil is changed hot or cold - if that's what was meant. Even if oil is drained at 200F, the same film of oil is going to remain on the parts that might become "varnished". The oil film left over on the valve train, valve covers, etc is the same every time the engine is shut off, regardless if the oil is drained hot or cold.
But the contaminants aren't even IN the oil when cold, making it impossible to drain them. That is if the contaminants exceed the holding capacity at said temperature.

The little bit that's in the oil that's left behind easily gets absorbed by the new bulk oil, the stuff that dropped out, who knows. Could be enough to saturate the new oil very quickly.
 
Messages
27,234
Location
PNW
But the contaminants aren't even IN the oil when cold, making it impossible to drain them. That is if the contaminants exceed the holding capacity at said temperature.

The little bit that's in the oil that's left behind easily gets absorbed by the new bulk oil, the stuff that dropped out, who knows. Could be enough to saturate the new oil very quickly.
My thinking is when you shut down a hot engine, all the oil drains off of everyting and into the pan or pockets within the heads, etc. leaving the same oil film on all parts regardless if the oil is drained when still warm or drained later when cold. Guess I'm not following your varnish comments - ?
 
Messages
5,379
Location
down in the park
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.
 
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