Engine Oil – Viscosity

Whew, that last bit was kind along winded, don’t you think?   Anyway, we’ll move on to viscosity.

What is Viscosity?

In a nut shell, it’s how easily the oil pours.   It’s a simple measurement of “thickness”, “pour-ability”, or “weight”.   Pour an ounce of water from a glass.  Now pour an ounce of maple syrup.   The maple syrup has a greater viscosity than water.   Simple, right?

There are various ways to measure viscosity.   And the charts are different for different kinds of lubricants.  You can’t compare motor oil to gear oil, it’s a different scale.  Once again, Google is your friend.

As you know, fluids pour differently and various temps.   To standardize things, it’s generally agreed that lubricant viscosity will be measured at 40*C(100*F)  and 100*C (212*F).

If you look at the SAE viscosity charts, you’ll see that a certain viscosity of engine oil can actually cover a large range of actual kinemetic viscosities.  A 20 weight oil ranges from 35 to 75 on the kinematic scale.

Winter Weight

What?  Winter weight?  it’s a race car, who cares about winter?  Unless you’re ice racing, of course.   Actually, it does matter rather a lot.

The winter weight rating – 5W, 10W – is important.   It’s a measurement of how well the oil flows when the engine is first started.   That’s when a large amount of engine wear occurs – when the engine is cold, the parts are moving, and all the oil is in the oil pan, not doing any good.   The thicker the oil is, the harder it is to get moving, and the longer it takes.  Which means more wear is occurring.  That’s especially bad on a high pressure valve train.

5W doesn’t mean the oil will have a viscosity of 5.  It means that it will pour like a 5 weight oil that has been cooled to 32*F.   Again, there’s that standard for measurement.

Also, remember that the viscosity is not 5 or 40, it’s a range between them, mostly dependent on temperature.   Many engineers argue that you should use the lowest cold viscosity you can find.   Even if the oil temp is 65*F when you turn the crank, that lower viscosity will lubricate the engine that much faster.

In the two charts above, compare the viscosity at at each temperature point.   Remeber that 24*C = 72*F.    Have you ever started your race engine when it’s freezing out there?  I have.  I don’t want the oil viscosity to be over 1,400!   I don’t want to be pumping grape jelly in there.

Above charts are shamelessly stolen from here:   http://www.viscopedia.com/viscosity-tables/substances/engine-oil/    Shhh, don’t tell them.  😉

What Do I Need?

Again, it depends on what you’re doing.   Generally speaking, you need enough viscosity to maintain proper oil pressure in all situations.   More viscosity is not always better.  Thicker oil requires more energy to move around, moves a little slower, and puts more strain on those moving parts because of the higher energy requirements.   If you make good oil pressure with a 30 weight oil,  changing to a 50 or 60 weight oil is detrimental to your engine.

There are some excellent spintron studies that show a 40 weight oil is better at stabilizing the valve train at high RPM’s.    For racing, I use 5W-40.  For my street cars, I use 0W-30;  I often drive my street cars at temps well below 0*F.     Unless you’re racing in really high heat situations – like desert truck racing – there’s really no need to go more than that.