Overclocking the Core i5-8600K is relatively easy due to its unlocked multiplier. All you have to do is set the desired multiplier in the BIOS and you're good to go.
It seems that our Core i5-8600K is not one of the lucky ones. Reaching 5 GHz is out of the question, not even with watercooling, no matter the voltage. 4.9 GHz is doable on decent watercooling, but it took a lot of extra voltage to fully stabilize every single application. 4.8 GHz, on the other hand, was relatively easy and achieved at a voltage level air cooling can deal with (to keep load temperatures below the thermal throttle point).