Universal Clays and 20/28 are two of the cleaner, better powders for 28 gauge. I use Universal, about 13.5, with a AA clone wad, Nobel primers (a direct sub for Winchester) and 3/4 oz shot. I prefer the AA to Remington in 28 hulls as they last a lot longer.
1oz loads are done with the 28, but in reality, a 1oz 20, 16, or 12 would pattern MUCH better. Winchester should make a factory loading. You might find components at BPI.
You can also ask over at the 28 gauge society forum