Changing Oil: Cooling Times

[llll1l1ll]

I've always wondered this:

When I change my oil, I get my engine nice and hot to get the oil flowing viscously in order to aid in the draining of possible sludge.

The problem is, I don't want to add oil that's at ambient temperature to an engine that is at operating temperature.

I usually wait about an hour or two before I even add the oil. I was just wondering what r3v did.

Best,

Pete

[MikeDE]

I've done hundredes of oil changes at my old job and never had a problem with putting ambient temp oil in hot engines.

[jlevie]

I for one won't drain oil or coolant from a hot engine as that can result in scalds. I'll allow the engine to cool to the point that I can keep my hand on the head or pan, depending on what I'm doing. In the case of an oil change, the oil is more than warm enough to flow easily and completely drain.

Whether you allow the engine to cool or not no damage will result from putting cold oil in a hot engine.

[Sagaris]

I let the car sit for 20-30 minutes to let as much as possible drip down to the oil pan and letting it cool down to a temperature that isnt going to cause me too much profanity when I touch it. Then I drain it and fill the oilpan up completely with engine flush and idle it with that for 5 minutes. KIDDING , dont taze me bro!. It usually takes me a long time to get the filter situation worked out so by the time thats resolved I just add it. I dont believe that there would be any significant thermal shock from adding ambient temp (based on a hundred years of people not paying this kind of attention to how they change their oil).

[llll1l1ll]

Yeah, at Jiffy Lube they just dump the oil on in. But that's Jiffy Lube, and they are sketch.

Dunno, it just seemed like it would stress the metal in that one corner of the head where you pour the oil in.

[nando]

ah, the beauty of ceramic coated headers. after 5 minutes they're warm but I won't burn the shit out of myself if I accidentally touch one.

Anyway I always drain it hot, but I usually do other stuff (BS with friends, drink a beer) so by the time I get around to filling it it's cooled anyway.

[llll1l1ll]

That's what I do. I just take my time (usually trying to finagle that oil filter outta there without getting a new scar) to make sure I do the job right.

By the time I get to the adding oil, the car has cooled down a bit - but not a whole lot. That's why I worry.

[AlarmedBread]

Use a long magnetic extension on the drain plug.
Use gloves when you get the filter.
No reason to wait around really, it's not like you're pouring in cold water which is a good conductor of heat.
I guess you could spend the time emptying your oil cooler once in a while. ;)

[SpecM]

dude, don't worry. You're not hurting anything added "cool" oil to a hot engine.

[llll1l1ll]

I didn't think so, but I wanted to know so, lol.

[stumble_t]

On a side note, It seems counter-intuitive to drain oil when hot.
With modern multi-weight oils viscosity increases when it gets hot, flowing less. Or am I missing something?

[supra89t]

On a side note, It seems counter-intuitive to drain oil when hot.
With modern multi-weight oils viscosity increases when it gets hot, flowing less. Or am I missing something?

From How Stuff Works:
"At cold temperatures, the polymers are coiled up and allow the oil to flow as their low numbers indicate. As the oil warms up, the polymers begin to unwind into long chains that prevent the oil from thinning as much as it normally would. The result is that at 100 degrees C, the oil has thinned only as much as the higher viscosity number indicates. Another way of looking at multi-vis oils is to think of a 20W-50 as a 20 weight oil that will not thin more than a 50 weight would when hot."

So, the actual viscosity is lower when oil is hot, but the hot oil acts like a hot version of a higher viscosity oil. Confusing, I know. Short answer is that the oil is thinner and flows better when hot.

[Wh33lhop]

On a side note, It seems counter-intuitive to drain oil when hot.
With modern multi-weight oils viscosity increases when it gets hot, flowing less. Or am I missing something?

Yes, you are. Why would you want oil to be thinner when cold? Ideally it'd be the same viscosity at all temperatures. "Multi-weight" oils just lessen the thinning effect when the oil gets hot (or rather, lessen the thickening effect when it gets cold).