OK, as stated above car oils contain friction modifiers to increase mpg figures. These friction modifiers will bugger the clutch. So using car oil is a big NO.
Also motorcycles shear the oil more than cars, so the oils has to be more shear resistant or it would go very thin very quickly.
Due to how the 'lines' are drawn as to put an oil on the SAE 30, 40 or 50 group means that some oils labelled SAE 30 and SAE 40 could actually be closer in terms of viscosity than 2 oils that are both labelled SAE40.
Basically an oil at the viscous end of SAE30 could be very similar to an oil at the least viscous end of SAE 40.
Much closer in fact than 2 oils that just fall into the SAE 40 category but are at opposite ends of the scale. (one could be almost SAE 30, the other could be almost SAE50).
The 'W" number is the viscosity cold and for oils with numbers of 5W or 10W it is measured at temps between -20C to -35C. I certainly wont be starting my bike at those temps so I do not believe they are as important.
As far as I am aware the only MOTORCYCLE specific 5W40 oils I can find are POWER1 racing and the one branded Halfords.
I do not believe for 1 minute that Halfords produce this oil.
What is special about POWER1 Racing 5W40? The only place I can find to buy is BMW main dealers in 1l bottles and therefore the charge what they like.