If you were getting some blowback past the rings I would expect the oil to be slightly darker than the norm, did you reinstall the plugs run it again and recheck?
Although a lycoming have a read
https://www.cessnaflyer.org/maintena...intenance.html
https://www.cessnaflyer.org/maintena...intenance.html
https://generalaviationnews.com/2012...ug-oil-fouled/