I have found Klotz to be fantastic oil. I use it exclusively and have always had good results. I'm not a big fan of castor. I can't say for certain why your plugs are discoloring but castor, in general, will varnish/carbon up an engine. I know dis'in castor is going to ruffle feathers, but love or hate castor, I think we can all agree it turns things black.

From what you describe I would say your engines are healthy. I have found that no mater what fuel I run, the plug body often does not stay shinny new. The element will often go from shinny to gray and sometimes, very near the plug body, it will accumulate a black deposit. All these are normal.

Pictures would help.

As a side note, the other day I was testing some new fuel dye. When I test a new dye I add a lot more than I would for a normal batch and then ran the engine a few hours on the test bench to check wear. The idea is that with 10 times more dye than normal, problems should show up sooner. Attached are a few pictures. Your piston doesn't look like this does it? It is interesting to note that the pink is dying the oil film left after running and this piston washed clean with a little methanol. It actually ended up showing where the oil film is thinner.

Bill