oil: why change it hot?

[Tom239]

I was always taught to change oil with the engine warmed up,
and indeed the first step of the oil change instructions in the NSX
shop manual is "warm up the engine."

A 1991-1996 NSX engine takes 5.3 US quarts at an oil+filter change,
and 6.7 quarts at an engine overhaul. If I'm not mistaken, that means
a typical oil change leaves 1.4 quarts unchanged inside the engine.
In light of that, getting a few more drops of oil out of the pan (by doing
it with the oil warmed up) doesn't seem to make a meaningful difference.

Is there some other reason for warming up the engine first?

 

[gobble]

The dirt is suspended in the oil just after the car is run. If you let the oil cool, the dirt settles and might not drain out.

 

[MarkB]

Additionally, warm oil will drain much more readilly than cold. With a cold block, you could let your car drain for a couple of hours and still not get as much oil out as you would by draining with a hot engine for 1-2 minutes.

 

[Tom239]

Additionally, warm oil will drain much more readilly than cold. With a cold block, you could let your car drain for a couple of hours and still not get as much oil out as you would by draining with a hot engine for 1-2 minutes.

That's the explanation I've heard over the years, and it's the one I'm skeptical about. Is it a big deal to leave 1.5 quarts unchanged instead of 1.4?

gobble's point (about suspended crud) makes more sense to me.

 

[EIFFEL]

Cold engine makes incomplete oil change, too much junk gets left behind. When I replace oil pan gasket on cold engine, I am surprise to see all the crud left at the bottom of the oil pan, same job on a hot engine, much less of the concentrated junk are left behind. BTW, I much rather work on a cold engine anyday! :smile:

 

[DRZZZ]

Gobble must have had the same auto instructor that I had. I was told to get as much "stuff" in suspension by heating and stirring the oil (ie. warming up the engine), and to change it right away. I've never seen hard data to substantiate it, though I still always do it that way. :wink:

Dave

 

[NSXTech]

The hotter it is, the thinner, the faster it flows. The flow helps sweep along more particles, not suspended, i.e. the stuff that collects at the bottom of the pan.
Just another reason.

MB