LVDell

So maybe this is a stupid question but it is one I don't know the answer to. Figured I'd let the collective braintrust here enlighten me on the reasoning.

I have been changing the oil in my P-cars for almost 10 years now and always let the car get to operating temp like everybody suggests and then do the oil change. For some reason I can't figure out why I have never questioned this.

So......why would you want the oil warm/hot instead of cold which would have maximum drainage into the sump/tank from sitting for an extended period of time? I can't imagine it won't flow well cold so I'm actually stumped on this one.

Thanks for any explanation!

Quadcammer

Generally quicker flow. You'll never get all the oil out anyway.

Some also argue that if you put your car on a serious slant front to rear, with the oil warm, you may be able to get some oil out of the front mount oil cooler.

Dock

I prefer the oil to be "warm" when I change it, not "hot".

Mikelly

When the oil has been heated, it flows better through the oil passages and tends to drain more thoroughly, and more quickly...

At least that's what I read somewhere many many years ago...

Mike

LVDell

Cool, thanks Mike.

So do you think that more oil would drain from a car that has sat for 2 weeks and hasn't been started (i.e., drain cold without starting) or would more drain if cranked up and drain after the engine has warmed the oil up?

Mikelly

Dell, at the risk of starting some nonsensical debate with the "non-experts" here about such things, I'd have to say that you would stand a much better chance of oil not fully draining, due in no small part to the oil pouring thru the passages like honey kept in a fridge... But that's my un-educated guess from years of wrenching and not being an "educated non-expert" Ungineer.

Mike <ducks and heads for cover>

LVDell

duck and cover, duck and cover

No HTwo O

I always thought the hot/warm oil carries away more of the "dirt" vs the cold oil, too.

grayman

Oh boy, I could hardly wait to jump into another oil debate..... just kidding.

Mike, that item we discussed (and a couple of other things) should be on their way to you next week. Let me know when they arrive.

LVDell

That actually makes a lot of sense. Thanks for the input.

Trust me, I never intended for this to be a debate. Just trying to understand why it is preferred to drain oil heated up rather than cold. I totally understand that heated oil will drain faster than cold oil but what I don't get is how much oil is left behind that circulated just a few minutes prior when heating it up.

grayman

Just making a little joke Dell
Good question though. I have heard the same advice many times, and I always thought the same thing, that warm/hot oil will flow out into the pan better. Interesting thought though about warm oil possibly carrying more gunk with it. Hmmmm. I don't do any of my own wrenching (with my mechanical skills, it would be dangerous to ride in a car I work on). I'll have to pick my mechanic's brain in the spring and get his opinion.

LVDell

I know. I should have added the little smiley face as I know how these things can take a life of their own and I truly want to know why heated is better than cold.

No HTwo O

Well, if warm/hot oil flows faster than cool/cold oil, it only make sense to me that it would carry more of the bad stuff (dirt) away from the vital engine parts, and NOT get hug-up someplace. It is also possible that the "dirt" is more soluble in the warm/hot oil, and gets carried away more thoroughly too.

raineycd

So doesn't all the oil go to the bottom of the case or the pan (in normal cars) and the sump doesn't have anything to drain from when you turn the car off. So in theory the oil is already in the bottom of everything when it is cold anyway and you would get more oil out since it has drained longer?

Unless there is more oil in the system on these cars that makes it fill up above some ports and it would have to drain out. Otherwise, I think a cold change on a car with a pan would be better since the oil is completely drained into the pan.

Would also depend on the viscosity and condition of your oil as well... I used to be so **** I would pour a quart of oil through the engine and let it drain to get all the old out! lol

Macster

The amount of oil -- cold or warm or hot -- that drains is about the same, near enough to not make any difference. I'm not even concerned about changing my Turbo's oil with the rear of the car up on the air on ramps. The bit of unlevelness just doesn't make any difference in the amount of oil that drains.

There are several benefits to having the oil warm or hot. Of course I have to mention the obvious and that is the hot oil presents a burn/scalding risk to the the person doing the oil change.

One benefit and I think the most important is if the oil is warm or hot the old oil and its contaminates are mixed/blended well and when the old oil drains away it carries more of these contaminates with it then were the oil cold and the contaminates having had time to separate.

If the oil is allowed to cool completely down, the water and unburned fuel separate.

The water seeks the lowest points in the engine oil tank, crankcase and everywhere else the oil sits and puddles. While the water in the crankcase and tank drain out a larger amount of water remains behind in the engine. This water of course contaminates the fresh oil.

However, I do not know for sure if the amount of contamination that the new oil is subjected to is significant. But one I think wants to err on the side of caution and draining the oil hot is better in that it results in less contamination of the fresh oil with the residual oil and I believe the higher percentage of contaminates left in the engine that would otherwise be there.

Two is the process goes quicker. Not only does the oil drain quicker, but upon refilling one doesn't have to run the engine as long to warm up the engine and oil enough to check the oil level.

Sincerely,

Macster.