The copper Washer used on the Plug from the factory is also the same as those used on the Oil Pipe (to the Oil Filter) Banjo fitting, I wonder if that's why they used them?
I did try Helicoils years ago and had limited success with them, and many others I've come across also seem to always have a drop of Oil on them. A few also pulled out, sometimes leaving things in a worse state.
These days, I make and fit an insert, and so far, been very happy with them, only now I've gone away from the copper washers to the fibre types