Grain Filler

I am working on a kitchen/dinning table made with red oak. As all you pros know, red oak is hard but has open grain. From what I have read on the web, I should be using a wood grain filler if I want a smooth table top. I tried a slurry on a test piece which was a disaster. So I decided on Behlen oil based natural wood grain filler because all the literature I read thought it was a great product. I'm sure the product is fine but my problem is how to use it correctly. The method I use is 1) apply oil based stain after sanding to 220 grit, 2) apply light coat of top finish (polyurethane), 3) apply grain filler. Step 3 is the killer. First experiment was to thin the filler just using the stain but it dried to fast. Second try was to thin filler with mineral spirits and add stain. With this mix, I had a filler with a consistency of heavy paint and it went on very easy. Let it dry to a haze, wiped with a clean cloth and allow it to dry overnight. However, once dried, the filler, which is gray in color could be seen in the grain. Basically, it would not take on the stain color, no matter how much I added to the filler. The filler lightened the stain color. Also, the filler did not completely fill the gain which would mean another application. This is my first try at filling grain so I'm sure there is a better method. I am leaning to my 'old' way of finishing a top by staining and then using multiple (many) coats of polyurethane (sanding between coats) to get a smooth, hard finish. Sorry for the long post. I think I'm missing something and making this a much more difficult task than it should be..
Thanks for any/all help on this subject….