Engine oil cold/hot change!

[DIYspanner]

I appreciate when having warmed the engine, all contaminants have been stirred up into the oil and flush out when the sump is emptied.

Are there any other advantages to a hot oil change compared to done when cold?

 

[Burnerman]

Everything happens one hell of a lot quicker, and if you are using a suction device to draw out any dregs from oil coolers or whatever it's much more successful.
John

 

[DIYspanner]

thanks John!

I'll keep doing the hot changes then.

Its confusing because a 5w30 is thinner when cold.

 

[JohnD]

a 5w30 is thinner when cold.

No it isn't

Where did you hear that?

A 5W30 is as thin as a cold SAE5 when it's cold,
and as thick as a hot SAE30 when it's hot.

But a cold 5 is thicker than a hot 30.

 

[DIYspanner]

As I said, confusing!

So the prefix number does not represent the scale as the suffix number on a 5w30?

Regardless of engine temp, viscosity is viscosity.

 

[JohnD]

viscosity reduces as the oil gets warmer.

So yes, a 5Wnn oil, tested cold, has a viscosity in the same range as an SAE5 oil when cold.

When you heat it, it gets thinner.

But additives prevent it getting as thin as a hot SAE5 when hot.

Multigrades have their viscosity measured when cold and when hot. The first number shows their cold viscosity, and the second number shows their hot viscosity.

If you measured a plain SAE5, it would have a 5W5 rating but I don't think they are ever described in that way.

 

[DIYspanner]

If you take a 5w30, is the 30 number an SAE30?

 

[JohnD]

when it's hot, the viscosity is in the same range as a hot SAE30.

 

[DIYspanner]

So for a5w30, SAE5 when cold, SAE30 when hot and a SAE5 is thicker cold than a SAE30 when hot.

Understood, thanks.

Here's a cracking good video.