No you don't!
Originally Posted by bwbloch
Like I've already said in this thread, changing the ownership or permissions of system files and directories is a quick way to break your system. You shouldn't do that. Most of the stuff requires certain ownership and permissions to work at all.
If you don't have suitable permissions to copy a file to some location, you don't change the location's permissions, you change your own permissions. That's what "sudo" is for.
sudo mv somefile /usr/local/lib/R/site-library/somefile