Assuming a straight 30 oil could pass the test for the 20w rating, which it might (well, maybe with some pour point depressing additive), then one could make up a 20w40 multigrade by mixing equal parts of straight 30 and 20w50. That would halve the viscosity index improvers by spreading them throught the straight 30 part of the mix with the result of roughly a 20w40. Well, maybe there is more too it, but you get the idea.