Magic damage on autos?

Your auto attacks against turrets deal the greater of your AD or one half your AP, per the League of Legends Wiki.

"Damage to a turret is based on base attack damage plus the higher of bonus attack damage or 50% of ability power. When your attacks trigger Ability Power damage conversion (occurs when attacking a turret while your AP is more than double your bonus AD), the converted bonus damage is now magic instead of physical. Since Patch V6.9, turrets are now vulnerable to armor penetration and magic penetration."