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As with many things the answer is: "It depends". If the car will be operated in sub-zero temps at times and the highest temp expected will be no more than about 60F, I'd suggest 5W30...and I'd stick to Castrol or Pennzoil - just my personal preference. Then there are the synthetics.
I did a little homebrewed experiment a few years ago by putting small containers of Castrol 5W30, 10W30, and Mobil-1 synthetic 10W30 in the deepfreezer (5 degreesF) overnight, then watching as I poured them out. From that unscientific test I observed that the Mobil-1 flowed best, the Castrol 5W30 was notably thicker and the 10W30 more so. This is a well known property of the synthetics, so no surprise. They are much better in cold starting conditions.
Synthetic is 4-to-5 times the cost of petroleum-base oils so you need to be convinced of a definite advantage to justify them. We have a number of vehicles in the family, and I don't want to spend a lot of weekends under them and hauling used oil to the recycle depot. I use Mobil-1 10W30 and stretch the change interval to about 10,000mi. (The cars see mostly highway use in mild-to-hot weather in California.) Now that Mobil-1 is available here in the "Euro" SAE 0W40 rating, I'm changing all cars (and motorcycle) to it as their oil changes come up.
In my opinion, it's bad practice to mix types or even brands of motor oils. They each have an additive package which is designed to work with their own base stock to deliver good lubricating performance. You are trying to outguess the refining engineers when you mix them. Probably no harm done, but it's not optimal.
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Bob (81-244GL B21F, 83-244DL B23F, 94-944 B230FD)
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