Oh No, an Oil question
 

All this x66 demystifying got me thinking about how chemicals combine in my engine, specifically in the oil.

One of the little things I do is vary my oil viscosity with the seasons. In the winter, I use a 5-30 or 10-30 multiweight oil. In the summer I'll go with a 10-40 or 20-50 weight. Since I buy the Castrol Syntec which is on sale and stocked, I end up with quarts of all of these weights.
Up until the x66 debate, I never thought twice about topping up with whichever quart I had handy. So my oil is "blended" with regard to weights, not brand. In fact, I like making what I call Syntec blend plus, thats 4qts of full Syntec and 4.5 quarts of Syntec blend.

Does anyone think that is harmful and something I best avoid?

ML Dude,

Over the years there have been many myths and misunderstanding about oil, and I'm sure there are still many extant.

I used to believe that it was bad to mix brands or weights. In one of the zillions of oil related threads on this sight some many months ago, there was someone who had vast experience in a lab involving oils and additives who indicated that one of the SAE specifications that oil must meet is mixability(sp?). Although I still don't do this, and avoid it in everything I own, save for maybe the rototiller, I don't think it is harmful.

Good luck,

As far as the lubricity quality of a batch of mixed viscosities and/or brands, I don't think there would be a problem. The main concern might be one of whether the additives for anti-corrosion and dispersants are compatible.