Why is the dealership against me using 5w20 synthetic?

[CXTKRS1]

I stopped by the local Toyota dealer to ask the service manager if my warranty would be intact if a ran 5w20 synthetic instead of the 0w20 recommended in the manual. He told me no that only 5w20 dino oil can used and if I switched to synthetic I would have to use 0w20. I really cannot find a reason as to why I must use 0w20 especially considering the climate I live in.