How are different oil grades achieved? Is each one "engineered" differently or does the oil company simply thin or thicken things as required with addatives? For eg., I just put in the Belgian Syntec 5-40 last week and it basically seems like a slightly thinner version of the 5-50. They smell exactly alike too. Are these 2 oils basically the same basestock & addative package and Castrol simply thins one out over the other or is something else going on? Are the other grades of Syntec achieved the same way (other than GC of course)?