I have had my ads rotating between 4 that seem to do similarly well each. But when I set the Ad Serving to optimize it shows one ad about 8 outta 10 times....and it is commonly the ad with the poorer CPC. Whats up with that?
Set it to rotate evenly - don't let Google optimise. You are split testing, so each ad should get pretty much the same exposure as each other.
if you set it to optimize it will just default to the one with the highest CTR. There is no secret behind that button, it doesn't do any special magic tricks.
Don't less someone else control your stuff. YOU have to control everything by "Rotating" ads and see which works best for CTR + Conversion.