I have a 2000 Honda Civic EX (2dr) Both my left and right blinkers have started blinking 2x the normal rate, which I think generally indicates a bulb issue. However, the fast blink only happens when the headlights are on and just recently started with the left (was just on the right about a week ago). All 4 bulbs do work and do blink, just fast when headlights are on. I have not tested it with just the parking lights though.

Before I go ahead and replace the bulbs:

1. Could it be something else (I've heard relay flasher but don't know much about it)

2. Is there a considerable difference/preference between the Iridescent vs Amber bulbs (Sylvania 1157A or 1157AST)

 

This is probably a grounding issue. Check the grounds for the headlights and signal lights in the front of the car. You should have approximately zero volts from the outer shell of the bulb (still plugged into the socket) to battery minus under all conditions: headlights on or off, blinker on or off, etc.

I've never liked Sylvania anything. The amber bulbs are for cars with clear lenses in the front. If you have an amber lens already, use a clear bulb.