Fuel in the oil lowers viscosity of the oil dramatically!

Mind you if oil pressure is low with fuel in oil this may be the cause.
Bearings will no longer be supported properly under load. Lifters may depressurize and be the reason for the knocking when starting cold until "some" pressure can build to hold them in place.

The fuel will dilute the oil which will no longer stick to the bores. With a roller cam it will take longer to destroy the lobes but it will in due time. Rockers will not be too happy as well and produce more heat.

Find out if you have fuel in the oil - if "yes" than check where it comes from.