"Sealers" such as Permalyn sealer and Waterlox sealer are really no more than thinned finishes. If you're going to use Tru-oil, simply make your own sealer by cutting it with an equal part of mineral spirits. The purpose is to achieve the maximum penetration possible with the first couple of coats. There's no reason any sealer should muddy the finish.

If you're referring to fillers, I'm personally opposed to them.