I'd say you made the right choice. Built like a tank too, enjoy that baby. I don't know much about Gravelys. But the professional mower guys around here are on Toro Z-Masters. And my neighbor has one & also works at our local dealer, so I have mowed on his, but that one weighs I believe 1,100 lbs. Where mine is 500 lbs, & yours is only like 740 lbs.
The 89 Octane is helpful for your Kohler Confidant, since it runs over 9:1 compression ratio, & 100% gas is best anyhow. 4.4 hours, maybe you will get your early oil change yet this year.. On my Kawasaki it specs for 1st oil change at 8 hours, I always change the filter as well. Filters are cheap compared to engines.. I'm only at 63.6 hours since 8/21/14.
I don't think I'll actually hit the benchmark for the first oil change by December, but I'll change it nonetheless. I will also include a new oil filter. I'll sharpen the blades and clean it up and then store it for the winter.