Thread: Oil viscosity

Okay for all you tech heads out there - how come zinc in oil improves performance? Anyone?

Look Google "oil additive zinc" and you will find hundreds of explanations, and why too much is bad.
Regards Philip A

Originally Posted by Panda

Okay for all you tech heads out there - how come zinc in oil improves performance? Anyone?

ZDDP is used primarily as an anti-scuff/anti-wear additive. It's also cheap, and also poisons catalytic converters, hence it being reduced in level and dropped in newer spec brews.
Many people think that an oil needs copious amounts of ZDDP to be any good, particularly the flat tappet V8 brigade, but new/better ( more expensive) additives are being used such as Boron esters.
Oils like Mobil Delvac 1 have virtually no ZDDP yet that oil probably has one of the most robust anti-wear additive packages of any currently available oil.

Originally Posted by PhilipA

Look Google "oil additive zinc" and you will find hundreds of explanations, and why too much is bad.
Regards Philip A

That doesn't sound good! I'll have a look, thanks

Originally Posted by rick130

Sorry Frank, but that is just plain wrong.
No lubricating oil gets 'thicker' the hotter it gets. They all get thinner as temperature increases.

With the SAE viscosity range, the xW part refers to an oils pumpability at sub zero temperatures. This is actually measured as a pressure reading in centipascals (cP)
The 5W, 10w, etc refers to a specific range of pressures/temps the oil must meet at low temps.

As an oil gets hotter and thins out, its viscosity is measured at 100*C. These viscosity measurements are measured in a scale known as centiStokes. (cSt) Whatever viscosity our oil 'measures' at this temp determines which SAE viscosity range it falls into (20, 30, 40, 50, 60)

Remember that SAE xW-X numbers are not absolute, but a range that an oil falls into, and ALL of the final ranges are a lot, lot thinner than any of the W ranges.

The rate at which an oil thins is measured as its Viscosity Index (VI)
The higher the number, the slower an oil thins as it gets hotter.
Generally, the higher the VI the better, as it can be an indicator of better base oils (eg Synthetics usually have a much higher VI than a mineral based oil) but trickery can often be involved here as a blender will also use a range of polymers known as Viscosity Index Improvers that can artificially reduce the rate of thinning. These VII's are generally used with thinner type base oils to get a reasonable low temp pumpability, while still meeting the higher SAE high temp range scales. It's how a multigrade mineral oil is made.
Unfortunately VII's they have a tendency to shear in service (and more so in gearboxes, etc) and so over the life of the oil the high temperature viscosity becomes thinner and thinner.

Check out this Valvoline Synpower 5W-40 in TD5 for the relevant charts and some typical well meaning but misguided comments and answers.

Rick, thanks for your expertise in explaining oil viscosity, you undoubtably know your oils, but as I said in my post to Panda, I was "simplifying" the explanation in terms that most would understand, I'm sure if I had explained that Long Chain Molecules unwind and lengthen when hot and give better shear strength would have been a bit of overkill for someone that wanted a simple answer to which oil to use in her SIII, Thanks again for your detailed explanation, Regards Frank.