What would be better for breaking in a gas engine?

Ashless or Synthetic oil ?

Would breaking in an engine with synthetic oil damage the spark plugs ?


Thanks in advance!

Many people break in engines using synthetic oil using the same mix from day one on. There should be no issues with the plug.

I just use synthetic oil right from the start. No big deal. The high spots in a engine get worn down and polished until a oil film can form in between the parts. So it doesn't matter about the oil used. Mineral based oils will tend to leave more carbon deposits inside of a engine and on the spark plug. Synthetic oils tend to leave less carbon deposits inside a engine.

"Ashless" suggests to me Lawnboy. No one I know will use it.
Regards, Richard