[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?)

[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.