Mr Diy chain oil.

Visbella chain lube or something, blue color with white cap 1 , big bottle and cheap, put some news paper at the rim when spraying so won't overspray.

I do it once every month when I wash my bike. Or 1k km whichever comes first.

If you use z/x/o ring it wouldn't matter that much unless you run your bike in dirt or something.

Legend says gear oil (80w-90) applied thinly on the inside of the chain provides better rust protection, less fling, and less grime retention and provide just as good lubrication.

Most important is its cheaply accessible.