I am learning more than I need to know about bearings....

But the 6303Z really is not the part number for the bearing. It is a designation of size. The real specs of the bearing are the numbers & letters following the size number.
And worse, there is not a universal system- each manufacturer has there own system. And just when you thought it was bad, the complete part number may not even be printed on the bearing, just the size....6303Z

There are non-, single, & double: sealed (several materials) or sheilded bearings. Different bearing angles, greases and oils which all effect durability and rpm range...

You have got to find a detailed spec page to get the real story on a specific bearing.

Who would have known....