Originally Posted by Baddog
The RP did cure a halt in the engine. Not sure what it was but around 4k RPM's it felt like the engine fell flat on its face. I put in RP and it even feels smoother in the low end and doesn't fall flat on its face anymore.
I'm not convinced that's not just a coincidence. On my brother's 2001 Forester, a hesitation at 3000-4000ish was caused by a bad front O2 sensor. It's possible your O2 sensor was temporarily acting up and stopped at around the same time as you put in new oil. Unless you keep oil in way past when you should have changed it, or you use too heavy of a weight, there's no way brand of oil would affect a hesitation problem.