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Viscosity is the force required to shear the oil at
a certain speed and temperature. Oils work because they have viscosity;
the drag of a rotating part pulls oil from a low-pressure area into
a high pressure area and 'floats' the surfaces apart. This is called
hydrodynamic lubrication, and crank bearings depend on it. In fact,
a plain bearing running properly shows literally no metal-to-metal
contact. Experimental set-ups have shown that electrical current
will not flow from a crank main bearing to the shells. Also, the
energy loss due to friction (the co-efficient of friction) is incredibly
low, around 0.001. So for every kilogram pulling one way, friction
fights back with one gram. This is very much better than any 'dry'
situation. For example, the much over-rated plastic PTFE has a co-efficient
of friction on steel of 0.1, 100 times worse than 'ordinary' oil.
Oil viscosities are accurately measured in units called 'Centistokes'
at exactly 100 C. These fall into five high temperature SAE categories:-
(< means 'less than')
A decent quality oil usually has a viscosity that falls in the middle
of the spec, so a SAE 40 will be about 14 Centistoke units, but
SAE ratings are quite wide, so it's possible for one 40 oil to be
noticeably thicker or thinner than another.
So, a good oil must be quite low in viscosity even in the cold,
so that it gets around the engine in a fraction of a second on start-up.
On the other hand, it must protect engine components (piston rings
for example) at temperatures up to 300 C without evaporating or
carbonising, and maintain oil pressure.
When the polymer modified multigrades appeared, a low temperature
range of tests were brought in, called 'W' for Winter (no, it doesn't
mean Weight!). These simulate cold starts at different non-ferrous
monkey endangering temperatures from -15º C for the 20W test
to a desperate -35º C for zeroW (0W). So, for example, a SAE
5W/40 oil is one that has a viscosity of less than 6600 units at
-30º C, and a viscosity of about 14 units at 100º C. Now,
those who have been paying attention will say "Just a minute!
I thought you said these multigrade polymers stopped the oil thinning
down, but 6600 to 14 looks like a lot of thinning to me!" Good
point, but the oil does flow enough to allow a marginal start at
-30º, and 14 is plenty of viscosity when the engine is running
normally. (A lot more could damage the engine, so we don't recommend
the use of 24 unit viscosity SAE 60 oils.) The vital point is, a
monograde 40 would be just like a wax candle at -30º C, and
not much better at -10º C. It would even give the starter motor
a fairly difficult time at zero. (At 0º C, a 5W/40 has a viscosity
of 800 but the mono 40 is up at 3200.)
Another basic point about wide range multigrades such as 5W/40 or
0W/30 is that they save fuel at cruising speeds, and release more
power at full throttle. But complications arise…
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