As my last post: 0W30, 0W40, 5W30 and 5W40 are all equivalent to a 10 grade at 100 degrees C.
Well unfortunately that's ********. Firstly 10 grade motor oil doesn't exist. Under SAE J300 standard the SAE grades for motor oil start at 20 grade. Winter grades for motor oil (such as 10W) are measured by different testing methods to determine crankabilty and pumping (Cold Cranking Simulator and the Mini-Rotary Viscometer) only at low temperatures ( in the case 0W the temperature is -35ºC) and the the SAE grades 20 - 50 are measured for kinematic viscosity at 100ºC to determine their grade.
In a 5W40, the cold oil has crankabilty (cold start resistance to cranking) value of 5, and when at 100ºC it has the viscosity of a 40 grade oil. So when you tip 5W40 oil out of the bottle into your car exhibits the properties of 5W and when it heats up to 100ºC and thins out, it is a 40 grade viscosity not the equivalent at of some non-existent 10 grade.
A multigrade oil with a cold viscosity of 5W which has been tested at 100ºC and is found to have the viscosity in the range described as SAE 40 it is labled 5W40, one with a thinner viscosity at 100ºC in the range specified by the SAE as SAE 30, it is labled 5W30. To suggest that x
W30 and x
W40 are the same at normal operating temperatures is misleading. They are different.
W30 may provide some small gains in fuel economy and power over x
W40 due to reduced windage and pumping losses, but may thin out too much for mechanical protection in high stress and overheat situations. What is IMHO just as important as the grade of oil (making sure it is within the manufactures specification) is the Viscosity Index (VI) as this number gives you an indication of how resistant a particular oil is to viscosity change with temperature, i.e. its resistance to thinning out as the temperature rises. Higher VI is better
Anyway see Viscosity Charts - Bob is the Oil Guy
to get a visual on comparative viscosity