On 2 stroke race bikes there used to be a debate about the heavier oil adding ring seal leading to more power.

On the 1200 I use a heavier oil because the few extra ponies is not worth the risk to the lifters as mentioned.

I used Motul in the Showroom Stock Miata, one of the most noticeable things was the mileage increased nearly 2 mpg (in street driving) so I would think the horsepower increased as well.

Tom