That ASTM standard I linked says that it is similar to FED-STD-791/3470.1. FED-STD-791 is still current (apparently at revision D) and is an interesting read.Back in the day, pre synthetics, there was and likely still is a military spec that
required mixability for oils that they purchase. As most brands, at least US brands,
wanted to sell to the government, they made their products accordingly.