11-01-2019, 10:14 AM
Without labouring the detail too much, multigrade oils achieve the 2nd figure by means of polymers which have a tendency to break down as the oil ages. Thus a 20W50 will ultimately behave something like an SAE 40 when hot but is probably a bit on the thick side when fresh. A 20W60 (must admit I've never heard of one till now) will be that much thicker. A multigrade is definitely a better bet than a straight grade as it reduces the amount by which the oil thickens as it cools, thus improves oil flow on a cold engine, which is where all the damage occurs.