Yes, a very oil rich mixture could cause plug problems, even with an essentially lean mixture. The oily wet plug would come from the mixture refusing to ignite or incomplete ignition

Many moons ago we ran some tests for Castrol on a Yamaha 3XV. First on the brake and then on the track. Basically testing different oil mixtures (avgas and A747). Starting from 20:1 in steps of 5 up to 50:1.

On the dyno there was very little to choose between the ratios (jetting was optimized), but on the track you could feel the difference.

The 20:1 ratio kept cool, consistent engine temps, but seemed to lack midrange snap. The 50:1 was very crisp, but if used in a race situation we would have had to jet up in an effort to keep the engine cool on a long WOT straight, thus losing any benefits of the previous crisp response.

We ran around 30:1 at all races as this showed neglible engine wear with consistent performance. Hence why HRC recommend it I suppose!

I say around 30:1 as we would occasionally adjust the ratio if we needed half a jet size.