Switching to Synthetic oil can actually improve horsepower jut by letting the engine turn with less resistance. I didnt believe it until I saw it first hand in my old mustang. My friends and I were battling this same question so; we took it to the non-biased Dyno. We first tested the car with regular Ha***ine oil. It did 198 horses to the rear tires. That same day we switched to Mo***e 1 oil. Ran the engine to have the same engine temp as in the previous test. Ran it on the Dyno and got 201 horses at the tire!!! Three horses increase!!! That made me a believer. I am not saying that it will make noticeable horsepower difference but a little here, a little there...
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