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I assume your car isn't making alot of hp? Do you lug the engine for mileage?(3/4 throttle or more at 1800 rpm for example) Is the motor in good shape? not burning oil, or has any bad ticks or clanks, high mileage? Do you tow anything?
If all no, then you should be fine running almost any oil, but in your climate it makes more sense to use 10W30 to me, in general the closer the numbers the longer the oil lasts close to the original specs.
Switching both the engine and tranny fluid to synthetic should result in small increase in mpg as well. Without closely tracking mileage you probably won't notice.
Ian
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