Toyota Repair: Ran out of oil, now knocking, connecting rod bearing, piston slap

Question
91 Corolla 168k miles. I forgot to put the oil cap back on the valve cover after topping it off.  My daughter was later driving when the oil light came on.  She pulled over and stopped it but not before the engine was clattering.
I drained the oil, saw some evidence of metal shavings.  Replaced oil with 15W-40 plus 1 quart of viscosity index improver.  Mechanic looked at engine, listened to knocking, shorted plugs until he found #3 cylinder was the culprit.  He diagnosed as piston slap and said to drive it until it breaks, but don't bother trying to repair it.  I notice that immediately upon startup it knocks for 2 seconds before it quiets down.  I don't have a tach, but when accelerating through gears, knocking worst approaching high rpm, but disappears when reaching that gear.  At 45 mph, accelerating or decelerating, knocking will come and go with speed increase/decrease.  Higher mph, knocking constantly but quieter.  I'm guessing the connecting rod bearing.  Can I get access to this bearing by just removing oil pan?  If I consider repairing it, it would be only for my learning experience and maybe get a few more miles out of the car.  Is it worth it?  Any suggestions?  How high of viscosity can I go?

Answer
You may be able to save the engine or at least make it run longer, the rod bearings are normally the first thing to go when there is a loss of oil pressure, they can be replaced by removing the oil pan. The 15w-40 oil should be ok. You can try straight 30W but I don't even know if it's available anymore.