Here's an update. About three weeks ago, the car started squealing again at startup. Around the same time, I started noticed misfiring/hesitation under acceleration; I'm not sure the two issues are related since the squealing had happened for a couple years. I took the car in yesterday for diagnosis and one of the spark plugs was cracked. I had the plugs replaced with quality NGK Iridium's (vs the crappy Autolites the previous mechanic put in 50K miles ago). I also had their GDI Emission (injection cleaning) service. Lastly, I had Amsoil SS 5W30 and a Hyundai OEM oil filter (vs Bosch like last time) put in since it was about 3 months since the last oil change.
The car now runs like a champ. No acceleration issues or misfirings. This morning, I started the car cold and zero squeal and smooth idle.
What does this all mean? Not sure. I used Amsoil 3 months ago for the first time in this car. Is it possible it cleaned some gunk, dirtied up the oil and Bosch oil filter and started the squealing again? Perhaps the anti-drainback valve stopped operating well at startup?
I've heard the OEM filters don't filter down to as many microns as the Bosch, but they're higher flow. Perhaps that'll help?
Either way, this experiment continues and I'll keep the forum updated.