Last year, when I was changing the oil and filter on my tractor, I didn't have enough Rotella or Rotella synthetic on hand to complete the job. It was pouring of rain and I didn't want to go to the store so I just mixed the two together. I got to thinking and worrying about it so I emailed Shell Rotella and asked for their advice. Their reply was that as a company, they don't recommend mixing the dino and synthetic oils together but that they wouldn't recommend or suggest that I drain the engine and refill. I took that to mean that there wan't anything wrong with mixing the two types of oil and that they didn't "recommend" it as a CYA policy in case something else happened that could possibly be blamed on the oil. So, to set your mind at rest, it won't do any harm to mix them together in order to get rid of all the partial containers although it's probably best to keep to a single make and type.