As stated in the title, the question is simple, about formatting. Reading through physics papers, I have seen $\varepsilon_o$, $\rho_o$, $\mu_o$, to denote some physical constants. Note that all of them use the "$_o$" subscript (lower case o), but other autors use a "$_0$" subscript (number zero), for example, in $\aleph_0$.
Is there a consensus about this notation? Which one should be used? Are they both preferred depending on the field?
P.D.: I am spanish, so maybe I am not using the exact or correct terms to ask this question. I hope it is understandable.