Generally 30w oils which are ACEA A3/B4 or C3 are on the heavy side. These are your Euro spec oils (BMW LL01/LL04, Benz 229.50/51, Porsche C30, etc)
Redline maybe Amsoil has some oils on the thicker side for their weights.
Can anyone tell me what synthetic is "thickest"? I.E. highest range on the 30 side?
Judging by your Korean models in your Signature and probably needing SN Plus ....
Quaker State Ultimate
Rotella Gas & Truck
Google both for data sheets.
However, Thickest is not Bestest. If it were, both of my Korean vehicles would be using ^^those-2^^
In mid-priced ($23-$35) 5w30s I like - in no particular order....
Pennzoil Ultra Platinum
Valvoline Modern Engine
Castrol Edge EP
Mag-1 Synthetic (I found at NAPA)
It's my Fine-As-Wine 9...... without the need to take a loan-out, for five quarts of TGDI / GDI oil.