5W-30 or 10W-30?

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Oct 4, 2002
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CA
Toyota V8 4.0L: 11 years old with 90,000 miles
Toyota V6 3.0L: New with 2,000 miles

The owner's manual for both states either 5W-30 or 10W-30, with 5W-30 "recommended".

My assumption is that 5W-30 is recommended for all-weather protection. Where I live, the temperature is between 50F-90F year-round.

Does this make 10W-30 the better choice?
I'm pretty sure it does for the higher mileage engine. But am not sure for the new engine though.
I've bene using 5w-30 in both cars.

A couple of related questions:
1. Are VIs still a factor between 5W-30 and 10W-30 with SL rated oils? Or does it vary by manufacturer? The main reason to go with 10w-30 would be less VIs. But I read somewhere there's no longer much of a difference between 5w-30 and 10w-30.

2. I hear 5w-30 is more "fuel economic" than 10w-30. Would the difference in fuel economy ever be noticeable?

3. For a car that's not driven often (once every few days), is a thinner weight oil better?
 
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Well, shame on me for not searching more diligently. Please disregard the "fuel economy" question. And it seems 10W-30 is the superior choice despite Toyota's recommendation of 5W-30 over 10W-30.
 
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