logic 1 for c51 is till 5v, bt for s51 its till 5.5v

This illustrates some confoosion, let me try to clarify.

This is not the data you design to!

In the data sheet there will be "absolute maximim values" (maybe phrased differently) and those are the values the chip is guaranteed to withstand without being damaged. So those are there to tell you what you might have to protect against. An example would be the input to a A/D which should be protected by diodes to ensure that whatever happens with the input the max can not be exceeded. Also these values tell you why you never should let a wire from outside the box go directly to a uC pin.

Then there will be "typical" values ignore those

Then there will be Min and Max operating values, those represent a guaranteed performance of the chip. Those values are what you design with. As an example if a pin is guaranteed to produce the minimum high at, say, 10uA, you must make sure the load is no more.

Erik