yes absolutely right , simply stating that in e.g. 0W-20 means its viscosity index to remain at 20 on both -30 deg C and 40 Deg C above (Ambient). It is the most suited requirement of specific viscosity Index for the engine by the manufacturer which nobody is trying to grasp..... We have tried to correct the thinking but all in vein.
For 0W-20
OEM says its engine needs VI = 20 at -30 and +40 Deg C plus
For 20W-50
OEM says its engine needs VI = 50 at -7 and +40 Deg C plus
For 0W-50
OEM says its engine needs VI = 50 at -30 and +40 Deg C plus
Hence VI is different for diff engine as per OEM recommendations
Where as Lemans or Nascar is a different condition and hence oil needed is more demanding with shearing and evaporation factors too but if that stress level is not present in daily day driving than IMO it's not useful / implementable rather it is not good for the engine too in the longer run ......
Usually wrong grade oils after effects are felt at longer times...... so no need to drain your energy.
Now people here also says that the Manual is for JDM cars in Japan.....
It is clearly mentioned that e.g. most suited is for a specific Car model is 0W-20 but still if you are getting into hot region than as per OEM recommendations you can only go for 5W-20, 0W-30, 5W-30 and 10W-30 but when one thinks he is more Clever / Wise by putting 0W-40, 0W-50 or 10-40W / 20W-50 then it means he has started the slow poisoning by accelerating the wear rate.