U40 Cork Seal and Birchwood Casey Tru Oil gunstock finish are the two go to grip sealers. (Bichwood Casey is a linseed oil based finish commonly used on gunstocks, axe handles, etc.)
I've not used the Tru Oil personally, but I know guys that do. It is a linseed oil based product. I do use boiled linseed oil on my tool handles. Shovels, axes, etc. and love it for handles. Its so much better than the varnish that typically comes on handles. I sand the varnish off and apply linseed oil.
More than one coat of U40 is unnecessary. It can't penetrate after the first coat and you just get a buildup that starts feeling plastic like and lumpy.
You can reapply if you find its wearing off. Wash your grips in your preferred method. You can even do a light sand with 220 or 320 sandpaper to knock anything off. Clean with alcohol. And then reapply.
Yes, it does darken the cork a bit. But I do think its a richer look that almost has depth. I think it darkens cork filler a few steps further than the cork. I don't use cork filler in my grips so I'm not totally sure on that.
You can use any wood putty for filling your cork. Oak seems to be the best color. And, as already mentioned, you can take cork and sand it heavily and take that dust and make your own filler.
Otterods-High performance fishing rods