First of all, you want some form of manual tranny fluid in your trans. Motor oil just doesn't have the right additives.

Second, the weight should be 10w30, not 10w40.


As for the racing question... it depends. One race, probably not much. I assume they'd lose a touch of horsepower because some of them are likely using 0w30.

The problem comes in long term usage of "cheap" oils... over time they just don't take care of your engine as well as the more expensive stuff, which typically has more additives, is synthetic, etc. For temporary or emergency use you can use damn near any oil that is the correct weight and not have any problems.