You can run a task every 5 minutes by getting around a "1 task every 15 minutes" limit. The trick is to split your task into several separate cron jobs with longer intervals.
If now is 11:00
| header 1 | header 2 |
|---|---|
| 0,30 * * * * | starts at 11:00, repeat at 11:30 |
| 5,35 * * * * | starts at 11:05, repeatt at 11:35 |
:)