Nope. It'll make slightly worse power (you won't notice) and slightly worse fuel economy (you won't notice) at the benefit of a longer lasting engine. It even states in the (at least Canadian) owners manual to run 5-30 in hot weather, or whenever the car is being driven in any hard usage.
The 5-20 is good enough to get the car out of warranty, and an even 1% fuel economy gain just by changing oil is a MASSIVE gain for any manufacturer as it costs nothing extra, and 1% across millions of cars is a huge environmental gain they can claim. It's why you see so much 0w-20 being used these days. 5-20 was the older method, and polymer filler techniques have just changed over time to become more effective, allowing the use of the thinner base oils.
What I can factually say is that 15w-40 can be run fine, but you need to warm the oil a bit before driving hard, and that it'll increase startup wear. And I can say that after speaking to the racing tech support at Amsoil, they suggest 5w-30 for the 4G69 motor. HOWEVER, that is on a fresh motor. Worn motors, such as Pcfreak's, will see issues running thinner oils with oil consumption. A thicker oil can be a bandaid solution to lessen the oil consumption. It won't cure all issues though, eg, running a straight 50 weight race oil won't solve oil pressure issues, or oil consumption issues in a meaningful way, and will cause other issues down the line.
In the case of the 4G69 though, 5w-30, again, really is what the engine was designed to use. We share bearings with a 4G63, as well as crank dimensions, which they had to run 30 weight on to get it to survive reliably through warranty.
[getting a little off topic here: I'm unsure about the 1980s carbed 4G63 engines, but can speak to the 90s 4G63, and say that the claim of "modern engines have tighter tolerances and need thinner oil" is not applicable at all here, nor in the 4 cylinder engines Genesis/Hyundai/Kia produces right now in 2021. I've never seen it marketed as a claim, I've never seen evidence to support it, but it's an engrained myth.
The only truths I've found in my admittedly short 1 hour of research into the myth is that the tolerances in variation and tooling have been reduced over the years, and that piston to wall gaps have tightened up, reducing blowby. Neither of these are bearings/oil surfaces. Chevy for example, hasn't changed their V8 bearing tolerances since at least the 60s, or their V6/I4 since the 80s (that's as far back as the data I found went on both). Yet in 2021, they're calling for 0w-20 to be used on their engines, where as in the 60s, it was 10w-30, on the same bearing gaps. But hey, back in the 10w-30 days, those engines weren't eating their lifters like they do today. Even with sloppier machining, they survived. I found that 20w-50 was a common method on them for street use as well, but the warnings are, let them warm before running, and they'll run forever. That sounds just like the 4G69 on thicker oils. Just don't go too thick when abusing it, as you may actually create friction in the oil (irony) and it'll run hotter, and degrade quicker (source: Amsoil racing tech support)
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