Piston slap in mine (Renix) is mostly only noticeable at idle. Is your valve clatter, octane clatter? Does it happen when you are accelerating or at low vacuum? I've found a good oil, tends to help, if it`s just a slight valve rattle.
The wifes 96 HO, has never had any octane (or noticeable valve) clatter, but if it every did, I´d probably change to super. Or possibly go to a colder plug. Possibly go to the next thicker oil.
I've read a bit about piston slap, it seems to be a combination of piston to cylinder wall clearance and the natural harmonic of the motor. Or in other words, it's in many motors and you just don't hear it as loud.
I had a 4.2 YJ with a few hundred thousand miles on it and the pistons slapped like crazy. I was always dreading taking it apart and seeing scratches down the cylinder walls, but there weren't any.
Loud valve clatter in an old motor is usually worn parts, but to some degree seems to show up with low oil pressure also. Maybe the low oil pressure isn't related to the valve train noise, but just another sign of wear.
I just put a load of Castrol in my motor (it was on sale) and noticed everything got a lot louder. I was running mobil 1 10-50 (which was also on sale) and the motor was much quieter. The Castrol is just temporary until I get finished changing gaskets and stuff on the motor, then I´m going back to Mobil 1, but maybe 10-40 for the winter.
How many miles do you have on the motor?