It wouldn't be straight 30 weight then. If there were something added to a straight 30 weight (called viscosity index improvers) it has to be designated on the the label and would be considered a multi-viscosity oil, not a straight weight oil.
By definition, to meet SAE standards, 30 weight oil must meet all tests for 30 weight oils but MUST FAIL tests for 20 weight oils, for example.
You should also be aware that the numbering system 10w, 20w, 30, 40, etc. bears no relation to any specific test. The numbers merely rank oils in order of low viscosity to high viscosity without regard to any actual measurement. That is, 40 weight is not twice as viscous as 20 weight. The numbers simply make it easy for consumers to figure out what oil is thicker and what oil to put in their cars.
