Gaining power from oil is a real thing. Last year at PRI I attended a seminar hosted by Joe Gibbs Driven racing oils regarding how proper oil systems and how you can gain in power. It was a huge deal when they picked up 3HP by using a 0w-20 viscosity for racing versus the typical 30wt racing oil. Essentially you want to use as thin weight oil as you can that will still maintain your hydrodynamic wedge when it comes to a true racing application. You manipulate how thin of a weight oil you can use based off bearing clearances, operating oil pressure, coatings used, etc. Same can be said about using something too thick. You actually end up generating more heat and robbing the engine of power, not to mention causing unforseen issues like cavitation at your oil pump due to the viscosity at higher rpm. That will eventually result in a catastrophic failure from lack of oil pressure.
A high quality 5w-30 ester based synthetic will pick you up a couple hp for sure, I stay away from additives like this though. Don't worry about it too much beyond that until you are ready to build a true race motor with a dry sump!
Just my .02cents on oil viscosity haha.
Thanks,
Alex Goodwin
AlexG@motionlabtuning.com