It’s interesting. a lot of the info here conflicts with my experience. Shell M is a castor based oil, at least according to Shell. There are synthetic components, but it’s a castor oil. https://www.shell.com/motorist/oils-lubricants/advance-for-motorcycles/advance-racing-m.htm
IMO you can’t reliably get a reading worth making a judgement on from the main visible portion of the porcelain, especially with how little running plugs tend to get in racing kart. About the only thing I use it for is presence of detonation. Many “good” racing fuel will leave salt and/or lead deposits
I typically look at the mixture ring where the porcelain meets the body of the plug. You need an otoscope to look it it properly however, and I’d recommend one anyway.
There’s some good reading here: