I like royal purple, the factory is 10 miles away from me and sometimes they have it discounted vs the auto stores. Always used it in my truck, and I'm amazed by the beating it takes its still running like a champ, I can't say if thats the oil or just an extremely well built little 4.3.
Unless you're talking about racing engines or trying to get 150% of the original VE at peak torque, I wouldn't consider the differences too much. Always been told on the older engines to just use regular possibly with additives if reg itself doesn't do the job. I use penzoil, a sales rep/friend of mine spent an hour one day explaining how its better than other brands and he got me hooked.
...the research that I did between the two is a simple additive difference. The two biggest things I've read is the royal purple may have more build up in the bottom of the pan, which I can't attest to that after using it a while. Thats suspect to the molybdenum in royal purple, which I see as a good thing. Mobil 1 ran a test that showed less micro-scratches in their engine machinery in a test run, but it wouldn't be anything I'd be concerned about anyway
thought it was comparing mobil to purple for some reason 😳. Valvoline (and amsoil, quaker state) are both high quality full PAO oils, its just slight differences in additives and small portions of solvent. each has a very small strength or weakness thats hardly quantitative