Redline C+ is a great synthetic version of ATF +4. I know alot of jeep guys that swear by it. You can extend your drain intervals with the Redline C+, but don't forget the most important addition here, which would be a high quality atf filter of some sort. You can keep it simple and go with a magnefine, step up a notch and go with a Racor spin on (other spin-ons of lesser quality are also available, and there is always bypass filtration, which I am running on my vehicles.
Really anything is better than what you have from the factory, which is effectively nothing. If you go with a synthetic ATF make sure to perform a cooler line flush so that you are not wasting that expensive stuff. Drain and fills take alot more fluid to accomplish a 95% or so replacement of the old atf.
And to answer your question, yes Group III oils are refined petrol products. A true synthetic would be a Group IV/V which RL is. There is nothing wrong with Group III ATF's, although I have more faith in IV/V base ATF's and have had better results using them.