but sort of logarithmic: if you have a shelf with some 30 and some 10 and the customer says he wants 20, you need to put in two 30 and one 10 to get 3 qts of 20 (not 1 and 1/2 of each). It used to happen all the time at the Porsche dealers in the far north 40 years ago, where the factory said use straight 10 (multigrades were verboten, but that's what I used in mine, and got excellent wear) in the winter in that climate, but customers with their owners' manuals in hand would insist on 20. Of course, the interactions of multi-grade additives from two different types may complicate the picture even more.