Capability of RX8 BEC
 

I have a RX8 comb in my Summit.
I noticed that when I turn the steering the LEDs on board become dim temporarily.
The steering servo is Savox savox sc-1256tg (20kg, 0.15sec, stall current:5A) .
Is this an evidence of over current of the servo?
Should I use an external bec?

Could be binding in the steering mechanism. Do the bellcranks hit the end of their travel before the servo does?

Nope. I have checked that.

The RX8 has a max. 5A BEC output so the servo might be overdrawing the BEC for a short time. Try plugging a capacitor into the Rx. Something like 10V and 1000uF should help if that is the issue. If you don't feel like making one, there's the Novak Glitchbuster.

I've tried a capacitor of 4700uF (ASSAN, IIRC) and barely noticed the difference.
Maybe I will try to use an external bec.

If you try the external BEC, don't forget to remove the red wire from the Rx plug on the RX8. If that works, maybe you have a defective servo or a (slightly) defective BEC in the RX8.