A test of the unused oil would show what the actual viscosity is. A test of the drained oil would show you what the viscosity is after you used it. Both of those may be different than the published number.

 

This is what I found on the Mobil website:

Typical Properties - Mobil 1 10W-30

SAE Grade 10W-30
Viscosity, ASTM D 445
cSt @ 40ｺ C 62
cSt @ 100ｺ C 10.0
Viscosity Index, ASTM D 2270 147
Sulfated Ash, wt%, ASTM D 874 1.0
HTHS Viscosity, mPa不 @ 150ｺC ASTM D 4683 3.14
Pour Point, ｺC, ASTM D 97 -45
Flash Point, ｺC, ASTM D 92 224
Density @15ｺ C kg/l, ASTM D 4052 0.86
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Typical Properties - Mobil 1 5W-30

SAE Grade 5W-30
Viscosity, ASTM D 445
cSt @ 40ｺ C 64.8
cSt @ 100ｺ C 11.3
Viscosity Index, ASTM D 2270 169
Sulfated Ash, wt%, ASTM D 874 1.0
HTHS Viscosity, mPa不 @ 150ｺC ASTM D 4683 3.09
Pour Point, ｺC, ASTM D 5949 -48
Flash Point, ｺC, ASTM D 92 230
Density @15ｺ C kg/l, ASTM D 4052 0.80
------------------------------------------------
Typical Properties - Mobil 1 0W-40
Viscosity, ASTM D 445
cSt @ 40ｺ C 78.3
cSt @ 100ｺ C 14
Sulfated Ash, wt%, ASTM D 874 1.2
Phosphorous 0.1
Flash Point, ｺC, ASTM D 92 230
Density @15ｺ C kg/l, ASTM D 4052 0.85
Total Base Number (TBN) 11.3
MRV at -40ｺC 26242
Viscosity Index 186
HTHS Viscosity, mPa不 @ 150ｺC, ASTM D 4683 3.7
-----------------------------------------------
Typical Properties - Mobil 1 0W-30
Viscosity, ASTM D 445
cSt @ 40ｺC 63.1
cSt @ 100ｺC 11.0
Viscosity Index, ASTM D 2270 169
Sulfated Ash, wt%, ASTM D 874 1.0
HTHS Viscosity, mPa不 @ 150ｺC ASTM D 4683 2.99
Pour Point, ｺC, ASTM D 97 -54
Flash Point, ｺC, ASTM D 92 228
Density @15ｺ C kg/l, ASTM D 4052 0.845
--------------------------------------------------
Typical Properties - Mobil 1 5W-20
SAE Grade 5W-20
Viscosity, ASTM D 445
cSt @ 40ｺ C 48.3
cSt @ 100ｺ C 8.8
Viscosity Index, ASTM D 2270 163
Sulfated Ash, wt%, ASTM D 874 1.0
HTHS Viscosity, mPa不 @ 150ｺC ASTM D 4683 2.62
Pour Point, ｺC, ASTM D 97 -47
Flash Point, ｺC, ASTM D 92 228
Density @15ｺ C kg/l, ASTM D 4052 0.80
---------------------------------------------------

ISO VISCOSITY CLASSIFICATION SYSTEM
Many petroleum products are graded according to the ISO Viscosity Classification System, approved by the international standards organization (ISO). Each ISO viscosity grade number corresponds to the mid-point of a viscosity range expressed in centistokes(cSt) at 40 deg C.

For example, a lubricant with an ISO grade of 32 has a viscosity within the range of 28.8-35.2, the midpoint of which is 32.

Rule of Thumb: The comparable ISO grade of a given product whose viscosity in SUS at 100 deg F is known can be determined by using the following conversion formula:
SUS @ 100 deg F/5=cSt @ 40 deg C.

* As Injuneer pointed out these numbers are based on unused actual viscosity.

An interesting note is that 10w-30 is thinner than 5w-30 on both start up and at operating temperature. And 0w-40 is thicker than all on start up.