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Why do I have to warm up multigrade oil?

if multi grade oil is thinner when cold then why is most recomendations to warm it up prior to change

It gets the crud you want out of your engine into solution with the oil so it can be drained. All oil is like this, not just synthetics or multi-grade oils.

It’s not thinner when it’s cold than when it’s warm. However, it is thinner when it’s cold than a straight weight oil would be at the same temperature. In other words, the thickness varies less with temperature than a straight weight oil.

Again, these graphs seem to tell the story on relative oil thickness of SAE grades

0W-20 at room temperature (25C) is thicker than SAE 40 at operating temp (90C).

oil viscosity doubles for every 8F deg temp drop. oil at 45degF like in MN right now is quite thick. oil at 5F in jan is even thicker. oil pumps have bypass springs. almost all oil is bypassed upon cold starts. start car and drive easy for 2-3 min. if you can. ask my neighbor who stomps on the gas pedal EVERY time he leaves his driveway to go UP the hill to the stopsign on our road. he threw a rod in his mercury murader last yr. and everyone says the 48v marauder motors have such a stout bottom end.

The 32 valve 4.6L used in the Marauder has a very strong block, but the internals aren’t particularly strong. IIRC it has cast rods and a cast crank . I want to say that for N/A DOHC 4.6L applications. The 99/01 Cobras an manual Mach 1 had a forged crank, but cast rods. While the automatic Mach 1’s and everything else had a cast crank and cast rods. The 03/04 Cobras (supercharaged) had very stout bottom ends, with a forged crank, forged Manley rods, and forged pistons. There’s an interesting book on the subject of the 03/04 Mustang Cobra and the struggles with the bean counters that John Coletti had when designing the engine for the car.

I have a neighbor who took off right away with heavy throttle in his 1990 Accord.
After a few years he was leaving a trail of thin blue smoke behind when he did that.

You don’t need to warm up the oil first, but the oil is thicker when cold than when warm. If the oil is cold, it is all in the pan and has drained from the upper parts of the engine, but a thicker coat of used oil will remain on the sides and bottom of the pan.

If you warm it up to operating temperature, the oil will drain faster and less will be stuck to the sides of the pan, but a lot of oil will not have drained from the upper parts of the engine.

If getting as much of the old oil out, i.e 99% vs 98% is that important to you, warm it up, drive up on ramps, pull drain plug and remove filter while hot and let drain for several hours. This is really not necessary but if your that OCD then by all means, go for it.

Worse case, start cold, drive up on ramps and shut down while cold. Now a thicker film of oil will be left in all parts of the engine, not just the pan. So you may only drain 97%, but the good side of that coin is that there will be a film of oil still there when you restart after the oil change and the oil pump is pushing air for a few seconds.

Take your choice. Using an oil change procedure someone else may disagree with is many many times better than not doing the oil change on time.

Example 5W-30.

5W is the winter weight of the oil. This means the oil will have the viscosity of a 5 straight weight oil down to zero degrees F.

30 is the viscosity that the oil converts into as the engine reaches operating temperature.


Warmer oil runs o it of the engine faster when you warm it up.

So what does this have to do with this guys Isuzu Rodeo!!!


It was a response to @Cavell