Just seen on another article about using Redex overnight in the bores but to start it outside as it will smoke a fair bit.
If nothing was done to the head then it is feasible that one of the valves was "touched" and has been bent just enough to set up premature wear on a guide (I'm sure someone will correct me on this but I'm thinking it's possible). Gummed up rings would also be a possibility as oil would then get past. If you use Redex and it works I suggest that maybe an oil change soon after might be an idea.