As for the valve adjustments you have to take into account your inexperience doing that job. It may have nothing to do with oil. As for the oil. You might consider a high mileage oil, but do NOT put a thicken agent into the oil. I covered that in this video.
It’s easy to determine piston slap. All you need to do is disable the ignition to each cylinder individually. If the noise goes away when you disable the ignition to a particular cylinder you not only have found the offending cylinder, but you’ve also determined that you have loose parts in that piston causing the noise.
Scoring is common on the front bank of that engine. It’s mostly due to extended oil changes in my experience. I would not say that is what is responsible.
In summary, stick with the recommended oil and know that the issue is likely not debilitating and you should probably leave well enough alone before you get in over your head. Probably the only way to find the cause of the noise at this point is to disassemble the engine and measure all the clearances in the engine to find the ones that are excessive. That underrating would likely be more trouble than it’s worth.
Lastly, consider checking the oil pressure. If you find you have a fluctuating needle, this might indicated increased clearances in the bottom end parts. Here’s a couple of videos to help you understand that.
Good luck an as always keep us posted.