Find a better dealership with more intelligent mechanics. 0W40 is exactly what is recommended for the LT1/4/5 engines, which are the same family of engines that the L83/4/6/7 are from.Thank you all for your helpful comments. So I see that most replies see no issue in running 0w40 weight as I am doing right now. As it is time to change my oil now (6k miles) I went to my dealership and they warned me that using ANYTHING other than the 0w20 written in the cap would cause harm to the engine. They were shocked that I was using a 40 grade oil. No matter what temperatures they claim it doesn’t matter as “modern cars now use aluminum parts” which cannot tolerate heavy oil. It’s for Porche, BMW and high performance cars, not mine. Is this true? I’ve asked a few more people who agree with the dealer. I am CONFUSED now.
So far after switching to 0w40 I noticed my car performance better (a bit louder maybe when speeding) but overall I feel it’s fine. I was even considering running a 5w40 grade since it’s MUCH cheaper but now i am not sure.