I don't mean to perpetuate any myths but this makes a certain amount of sense to me. I have heard statements and read articles by apparent experts that say that the different chemicals used by different manufacturers as oil additives can sometimes have adverse reactions. Most recommend not mixing brands or types of oil. This could be accentuated by additional blow-by from an older engine. The lab testing is a good idea. However, I usually stick with one brand and try not to mix them. There is only a few cents difference between the name brands anyway (maybe more with synthetics). I also avoid oil additives for the same reason. I don't think they are generally needed.
Bill
