Nitro-based filler

Don't know, I think most DIY finishers on the board use water-based fillers. You can check it out on a any piece of scrap wood you have laying around; just apply a thin coat of the nitro filler, let dry completely, then put a coat of Tru-oil on top and see what happens... Never heard of anyone trying this before; know that if you shoot nitro on top of tung oil you get a mess. Make sure the filler dries completely, may work, as all the solvents that might jack with the Tru Oil should be evaporated.
Another filler i was recommended today at a wood finishing special store here in Poland is polyester filler (hope English name is the same or similar). It is not water based but they said it will fill pores and work a s a primer before high gloss polyurethane finish. What's your opinion?
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.