Naturally, monogrades existed first.
The different oils from different regions had different properties, some showed less viscosity change with temperature and were considered more desirable. So they were given a "viscosity index" of 100 (the best), and the worst given zero.
Synthetics had viscosity indices higher than 100 and were more desirable.
At some stage, viscosity index improvers were developed, and "multigrades' came about, that could cross the boundaries between a thin and thick oil...or add a lower "W" rating for a given KV100 basically.
These were ultimately found to no offer what they promised, and high shear viscometry was born.
Monogrades do not change viscometrically with the rate that the wear surfaces move past each other ... they are "Newtoian"...multis don't do that, they change with shear rate as the polymers "flatten out"...here's one of the early tests...
One of the "50s" perfomed as well as a 40, the other about as well as a 30.
That's why HTHS minimums were introduced into J300 (which has evolved continuously since the 20s), as it was found that HTHS was more related to bearing wear. (Ealy on, the 0W, 5W, 10W 40s were only required to have a 2.9 min, just like the 30s...that was changed recently to 3.5)...note, a mono 30 has an HTHS of about 3.5+