Insolubles means what? Solids that are undissolved? Cleanliness of oil? Is it related to Soot? If insolubles = 0.0 does it mean the oil is very clean? If insolubles = 0.6, does it mean the oil has poor cleaning ability?
Its how much grit and crap/deposits/solids is in the oil. Basically if its over .4 or .5 you should change it because it can be abrasive over that.
Someone else can chime in with a better answer now lol