Which came first, the engine or the oil?
 

Are engines designed around current oil weights, or is it the other way around? What got me thinking about this is the often debated issue of what oil to use in an older car. Can one use an oil weight that did not exist when the car was new. For example, let's say you have a '90 300E, could you use Mobil 1 0W-40. This weight, of course, would not be listed in that car's owner's manual, but could you use it? Of course the classifications would of changed over the years, like SG, SH, SJ, etc. So a W124 would maybe require SH, but that has since been replaced by SJ standard. It is fine to use an updated standard in anolder car, but again, what about a weight of oil that didn't exist when a car was new. In other words, if say 0W-40 existed in 1990, would a 1990 300E require it, as today's Mercedes do? BTW I hope this doesn't turn into another oil debate, I definetly don't want to be responsible for another "oil fire".