Total real frictional losses in a IC engine is about 6 to 8%. (REF: Mobile Labs, Lubrication Sciences, etc) So many wild claims of HP or MPG claims with synthetics are just simply bogus. Any HP/MPG gains only occur when the oil is cold, since the pour rate of synthetics is so much better. After 140-150F you will not see any gains with a synthetic.
Even if you could decrease frictional losses to zero (not going to happen) 6% is all you could ever gain in HP at best.
We have done many back to back tests on a pretty good engine dyno setup with dino oils and synthetics on SBF, SBC and ACVW's. After 150F not one additional HP is visible (or Torque for those that don't understand HP ).
Protection is the adavantage of synthetics.
If you want significant HP gains to the back wheels. Use the Synthetics in your drive train also. Trans and Diffs can eat up 10% of your HP. A synthetic can offer as much as 3+% more HP in the early laps. VERY measureable on the chassis dyno.
Ten+ years ago, Mobile Labs did dyno testing across many synthetic brands. They found NO difference in HP between the brands. They did find many oils to be better or worse in composition, QC, contaminants, etc. but not performance.