The nitro based sealer is a sealer, not really a filler. Its purpose is to minimize the effect of sink in - on the finish that is applied over it.
The poly sealer would be good - for poly finishes - but probably not for oil.
I hate to say - keep looking.
One unmentioned sealer, that works - is superglue. You can apply superglue, let it dry, and sand it off. You can refill the deep grain pores and recesses. It does in fact, create a very smooth and even surface if you sand it off with a sanding block (ALWAYS use a sanding block whenever sanding... regardless of the finish). So, at worst, you can get some commercial quantity of super glue and use that as a sealer/filler.
Another trade secret out for grabs..... yikes.