Just buy a Tridon HD12, which is an electro-mechanical type. It works on capacitor charge time, not on load resistance from the bulbs. That means its not sensitive to the number or size of bulbs you use.
With the stock type thermal flashers, if the idle drops off slightly at an intersection, allowing the voltage to drop down, the flasher would stop flashing, which is dangerous.
With the HD12, that won't happen. It will flash at 11 volts or 12 volts or 13 volts or 14 volts.
I've used the HD12 for about 4 years now and its works perfect with the stock #1156 bulbs.