1. Depends. The more often it misfires the more the loss of power.
2. Normal interval seems to work for pretty much everyone.
3. Why change the type of plug? Unless you have done major power increase work, (more then simple bolt-ons) it's unnecessary.
4. You need to find where the oil is coming from. Oring? Gaskets?
https://themotorguy.com/oil-on-spark-plugs/
https://www.ngk.com/learning-center/...ugs-heat-range