We are looking at getting an ISO 220 grade SGM gear oil for our Westfalia Seperator (centrifuge) and it lists certain spec in the manual. One of them is that the Corrosive effect on copper according to DIN 51759/100 A3, degree of corrosion 1. On some quotes we have gotten from Shell Omala VG220, Citgo SGM ISO220, and Amsoil SGM ISO220, they have different listings for the sopper strip corrosion. Amsoil is 1B, Citgo is 1B, and the Shell Omala Oil is 1a. What is the difference between 1a and 1b? Is "1b" better than "1a" for instance?