Funny I had the same thing happen, when I switched to a middle premium grade of Castrol. Thought I'd give the XJ a treat and give it a meal of the good stuff. The piston slap got louder and most everything else was noisier. I figured the new stuff was designed for mileage and not adhession. I've since switched back to regular old Castrol bulk oil. I use quality, middle cheap stuff, 10-40 ( for my high mileage motor) and change it often. Believe it or not, oil comes in batches and some are better than others. When I find a good batch, I buy a bunch. Or at least take note of the batch numbers. I have an electric thermometer, a viscosity gage and an adhession gage and occasionally test my oils. The best test is actually to rub various brands and mixes between your fingers (new and used).
I was running synthetic Mobile 1 (for decades in various vehicles and machines), but the quality has gone down and the price has gone up.
Piston slap is often RPM sensitive, a little higher RPM and it goes away. An oil that ahderes well to the cylinder walls, can lower the noise level some.
5-90 once said, the feds made the oil companies lower the adhesion index (additives) for many oils to save fuel or something like that. I'd noticed many top end oils just didn't adhere like they used to.
Additives are a pandoras box. The oil passeges to the the cam bearings and then to the main bearings are pretty darned small. An additive that actually improves adhession, without increasing the viscosity much or worse yet clumping and gumming up the works is advisable. Liquid Moly used to be pretty good stuff, but looks decidedly different than it did many years ago. It used to be almost black and would turn your oil black, but worked good. It left a micron thick coating, that was slicker than oil on all the internals. Worked great in noisey gear boxes and motors that would drain back to much and start up noisy.
I worked for years with pumps, industrial gear boxes and generators. You catch on quick to what works and what doesn't, they often run 24/7.