On relevance of engine oil HTHS viscosity values (ASTMD4683) to ALL everyday car drivers like you and me :
A) From ASTM website ,

ASTM D4683 - 13

Standard Test Method for Measuring Viscosity of New and Used Engine Oils at High Shear Rate and High Temperature by Tapered Bearing Simulator Viscometer at 150 °C

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5.1 Viscosity values at the shear rate and temperature of this test method have been indicated to be related to the viscosity providing hydrodynamic lubrication in automotive and heavy duty engines in severe service.

5.2 The viscosities of engine oils under such high temperatures and shear rates are also related to their effects on fuel efficiency and the importance of high shear rate, high temperature viscosity has been addressed in a number of publications and presentations.
http://www.astm.org/Standards/D4683.htm
B).................So HTHS is related to any engines operating in Severe Service.

Heck , what has this 'engines operating in Severe Service' has got to do with you or me ?????????????????????
C)Your car engine is subjected to 'severe service' ,  if any one of the following parameters applies to you or me in our everyday driving such as , among others :

a)driving at less than 6 km per trip, then shut down engine;

b)multiple engine starts and multiple engine shut-downs in a day;

c)driving under cold weather/raining days;

d) Frequent idling for long periods of time, such as stop-and-go driving in
heavy traffic. Many vehicles are subjected to this condition twice a day
in commuting to major cities...... KL/PJ/JB/Penang/Singapore folks be warned !!!!!!!!!!!!!!!

e)  Sustained highway driving in hot weather (sounds familiar ??), such as vacation travel.
f) Towing a boat or trailer, carrying heavy objects/loads on a rooftop rack/car boot.
g) Driving in dusty conditions, such as dirt or gravel roads. Kampung folks jaga jaga !!
h) Driving on steep hills or mountains on a regular basis.  Kaki Genting ? Orang Pahang ?
http://www.aftermarketsuppliers.org/Counci...lish/94-1R1.pdf
HTHS viscosity values has everything to do with your everyday driving habit ................... and Nothing to do with motorsports , in a way.
Folks , wanna better protect your car engines durability , check with ASTM/API on HTHS and its importance to your everyday driving.