^that is a common misconception about a 0wXX (0w40, 0w30, etc) oil
a 0wXX oil is really a 5wXX that has a lower pour point than 5wXX. 0wXX is able to pour at temps below -45C.
to achieve such a low pour point, Group IV and V synthetic basestocks had to be used...meaning that a 0w oil was indeed a true synthetic and not synthesized from crude oil like a Group III. Recent advancements have had Group III oils with lower pour points than -45C.
a 0wXX oil can be thicker in viscosity at 40C and 100C when compared to a 5wXX oil
Example:
Mobil 1 0W-40
SAE Grade 0W-40
Viscosity, ASTM D 445
cSt @ 40ºC
80
cSt @ 100ºC 14.3
Viscosity Index, ASTM D 2270 187
Sulfated Ash, wt%, ASTM D 874 1.2
HTHS Viscosity, mPa-s @ 150ºC, ASTM D 4683 3.6
Pour Point, ºC, ASTM D 97 -
54
Flash Point, ºC, ASTM D 92 236
Density @ 15ºC kg/l, ASTM D 4052 0.855
and, compared to Mobil1's 5w30
Mobil 1 5W-30
SAE Grade 5W-30
Viscosity, ASTM D 445
cSt @ 40ºC
64.8
cSt @ 100ºC 11.3
Viscosity Index, ASTM D 2270 169
Sulfated Ash, wt%, ASTM D 874 1.0
HTHS Viscosity, mPa-s @ 150ºC, ASTM D 4683 3.09
Pour Point, ºC, ASTM D 97
-45
Flash Point, ºC, ASTM D 92 230
Density @ 15ºC kg/l, ASTM D 4052 0.80
the supposedly thinner 0w40 has a
higher viscosity at 40C than the 5w30. notice that the pour point of 0w40 is -54C and 5w30 is -45C
Comparing the Mobil1 0w40 to
Motul X-Cess 5w40, the 5w40 specs are:
Viscosity, ASTM D 445
cSt @ 40ºC
92
cSt @ 100ºC 14.6
Viscosity Index, ASTM D 2270 165
Pour Point, ºC, ASTM D 97 -
33
Flash Point, ºC, ASTM D 92 230
Density @ 15ºC kg/l, ASTM D 4052 0.853
The Motul 5w40 has a higher viscosity at 40C but a lower pour point (-33C) when compared to Mobil1 0w40. At 100C, the 0w40 and 5w40 have very close viscosity ratings (difference of 0.3).
To summarize, 0w oils are not the worst you can do to your engine. They are not too thin to provide protection and do not become "even thinner" at higher temperatures.