My analysis indicates that the buy and sell level will vary somewhat depending on the amount of days used in the calculation. A shorter span of days will result in a more volatile indicator which reaches further extremes. A longer amount of days used in the calculation results in a less volatile reading which reaches extremes far less often. I have also found that despite the fact that RSI is designed to be able to measure multiple equities against each other, it doesn't quite work out that way. Some securities may pull back some when their RSI indicator reaches about 68, others at 70, etc.
Different securities seem to have slightly different levels at which the price changes direction.
These levels are close to each other. But they seem to be particular to each equity. Basically, the vast majority do seem to change direction at 30 and 70. My point is that this is not a hard and fast rule. There are subtle differences.