Oil change thoughts and observations, Ramblings of a greasy madman Options

[trackbird]

As I was laying in my 40 degree garage today, changing the oil in my Camaro that had been parked since the night before, I remembered that most people seem to recommend changing oil when it is hot, so it drains better.

Now, my question is.

If oil is thin when cold (for start up protection) and gets thicker when it heats up, is it really a good idea to change it "hot". It did seem to take a while for the last of the oil to drain today, but I'm not sure that it should have. It was just an observation, early today, before I really woke up. So, if it is thin cold, and "thick" hot, shouldn't we change it cold? Or should we?

Just wondered what everyone else thought.....


(Nata, if this "takes off", kick it to engines or I will, it's a "general question" for now, isn't it?)

[Teutonic Speedra...]

Trackbird,

You have the hot/cold backwards. Oil is thicker when cold, thinner when hot.

[sgarnett]

Actually, you're both right (IMG:http://www.frrax.com/rrforum/style_emoticons/default/wink.gif)

The base stock, even synthetic (to a lesser degree) thins out as it gets warmer. However, it may thicken again when it gets hot enough to break down, but that shouldn't be happening in normal operation.

The long-chain polymer additives that stabilize the viscosity expand or contract at high temperature (don't remember if they have a negative temperature coefficient). This has the effect of thickening the oil at high temperature, but it isn't enough to completely counteract the inherent thinning of the base stock.

It probably is possible to make an oil that actually thickens as it gets warmer, but it would take a lot of additives to get there, and it would shear down very quickly. That's part of why dino 10W40 isn't used much anymore - it didn't remain 10W40 very long in the engine.

[robz71lm7]

Pull up a MSDS on 5w30 10w30 or whatever you use. It'll usually state the viscosity at 40* C and 100* C in cst. It's thinner at 100*C. Oil thins out as it gets hot. Ever notice how you have lower oil pressure when your engine is up to operating temp?

The 10w30 label sorta says that it acts like a 10 weight would when it's cold and like a 30 weight would hot. They add viscosity index improvers (VII) to the oil so it won't thin out as much when it heats up. A lot of synthetics require less VII than conventional. Some claim to require none.

[trackbird]

I was thinking about the "thickeners" (additives) that are used to get the oil to "30 weight", etc. And thinking that if it was supposed to pump cold, and then thicken at higher temps, then it should be thinner when cold. But, it makes sense that it pumps "anyway" cold and stays "sorta thick" when hot. Like I said, ramblings of a greasy madman. I guess I was expecting more from the specs than actually occurs. I do usually change it warm, but yesterday, I didn't run the car first.

[zlexiss]

Let me try to add my version of it:

Multigrade oils still thin out when they get hot. But they don't thin out as much as a straight weight oil does.

So at room temp, a 5W-30 oil is as thin as a stright 5w, which is thinner than a straight 30w.

When hot, the 5w-30 thins out only as thin as a hot 30W, while a straight 5W would be a lot thinner.

So, you get oil thin enough to pump when cold, but doesn't get too thin when it's hot so parts stay protected (as film thickness decreases when the oil thins out)

-Jeff

[DryStout]

Now, my question is.

If oil is thin when cold (for start up protection) and gets thicker when it heats up, is it really a good idea to change it "hot". It did seem to take a while for the last of the oil to drain today, but I'm not sure that it should have. It was just an observation, early today, before I really woke up. So, if it is thin cold, and "thick" hot, shouldn't we change it cold? Or should we?

Just wondered what everyone else thought.....

Change it hot. There will be more contaminates suspended in the oil.

The guidance you got about the behavior of the multiviscosity oils is correct [hot thin, cold thick still applies]. It is all spelled out in the SAE standard. The low # is for viscosity behavior at a low temp, and the high # is the viscosity behavior at a high temp.

[NataSS Inc]

I have always changed my oil when cold. I just leave the drain pan under the oil pan for a little bit while I consume the next beer to drip for a while. IMHO the amount left in the pan is negligable.