If your pc is stable with 1.28v at 5ghz you are blessed with a golden chip. It shouldn't go over 65 degrees with that low of a voltage so yes, 1.28v is completely safe. Mine takes 1.275v for 4.8ghz and 1.375v for 5ghz to be stable.
I am prime95 stable for 12 hours with 5ghz at 1.296 voltage (although i've seen it fluctuate up to 1.312, I use offset voltage and level 1 LLC). So I think it's definitely feasible for a i5 8600k to get 5ghz at 1.28
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