If you hot lap / double entry on thinner oil in summer it can cause trouble and bearing failure ( local builder here --not me --has done it several times)

On the dyno-- you get oil Hot -- or run thinner oil it does make more HP--how much- who cares? what is 10 HP on a 700 HP bracket engine? It is a test that is all- so what it makes 10 more HP

Now on a circle track 350 two barrel with big rules limits and around 365 HP then that thin oil is the difference between first and last place

I have never had a failure that could be blamed on oil unless you consider flat tappet lobe lifter issues --NEVER a bearing or wear issue due to oil.
I have run every kind there is and it is like politics and religion and family matters--no good solid answers

With Nitro or alky it becomes a different tale--it DOES matter what you run due to dilution--some work / some fail big time in that dept.

Bracket racers might as well run what is on sale if they have a roller.