The calculators usually use $\log x$, while articles on wikipedia and other websites often use $\log_{10} x$, finally the ISO-80000-2:2009 uses $\lg x$.
Which one of these is correct? Or at least expected? If there isn't one, is any considered incorrect or discouraged?
Is the convention varied by regions perhaps?
Definitely not $\log(x)$. That is reserved for natural log. I would say use $\log_{10}(x)$.
Edit: This answer is more complete and correct. In computer science $\log(x)$ often means $\log_2(x)$. (Thanks also to @Qudit for mentioning this.)