Chain grade doesn't matter much. It will not be carrying a load at any point. Any cheap 5/16" chain will work.
You may want to consider galvanized if rust puts you off. Even then, the coating will be nicked and point of rust started there as well as where the chain is welded on. Paint will work for a while.
The reason I asked was that I saw an ad for an optional OEM chain guard for a particular mower that advertised the chains were held in place with an easily removable rod for replacing broken chains. So apparently they can break.
I was wondering that if maybe the reason they break was they were a certain hardness like a shear pin that would break before tearing up the bracket they're attached to.