I run Mobil 1 because I started using it about 10 years ago when I think it was the only readily available synthetic. But I wouldn't have any major heartburn about using
any other good brand. Change it regularly and change the filter at the same time. You'll be ok.
One other thing... some of the more expensive synthetic oils have higher levels of zinc. Zinc is a great anti-wear additive, but long-term it can accumulate in the cats and plug them up. Now whether or not that's an issue with the IS300 we won't know for a long time.
Mobil 1 has been "decontented" (less additives) over the years at the request of major manufacturers (also makes it more profitable for Exxon, that's their money-maker).
What am I getting at? I don't know.... well, it's just that having THE BEST may have long term side effects. Too many additives could be a bad thing.
I say just use a $4-5/qt synthetic (they're more similar than different) and change it on a regular basis.