Hi
Let me preamble this by saying I am no expert in Oil and Lub.
My question is what benefit(s) are you hoping to obtain by going to a 0w20? Not mileage as that only appears when you have 1000s of cars. Do you have an engine problem you want to address (noise?). My thinking is if all is fine with your EX, why second guess the highly trained engineers who developed and improved the VQ engine over the last 10 years, and know the engine inside out?
My second reaction is the the 0 weight (in 0w20) is actually better for cold weather, which your car will not see. So I think that one or twice using a lighter oil you might be fine, but I would not do so over the life of the vehicle, personally. There is also this warranty thingy that could be impacted if something goes wrong and they discover you have been using the wrong weight oil.
There is some debate on Bob Is the Oil Guy as to whether synthetic oils are better than good quality dino oil for normal OCI (which is 6.5km for us Canucks). I know our Infiniti dealership uses Mobil 1 in the EX and other Ester oils in some Nissans (albeit there is no mention of us having to use Ester oil or even synthetic oil anywhere in the EX owner manual). I use Mobil 1 5w30 because I trust it but could explore other synthetic if I had the opportunity and I find some on sales. I use synthetics because of our weather which can get very cold for long period of time.
Sorry I cannot be more of help. I would not do it.