Bearings with slots, or the ones with a groove all the way around the top bearing shell keep the oil pressure applied to the drilled hole going to the rod bearings a little longer. As long as the lower bearing is in full contact with the main journal, that's where most of the load goes while running. Slots or grooves on top just help the rods get more oil pressure.
Jerry

"It is better to be silent and be thought a fool than to speak and eliminate all doubt!" - Abraham Lincoln
Cringe and wail in fear, Eloi- - - - -we Morlocks are on the hunt!
There is nothing noble in being superior to your fellow man; true nobility is being superior to your former self. - Ernest Hemingway
Love your enemies and drive 'em nuts!