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If using non-synthetic:
10W30 for colder weather, 15W40 in hot weather or for a sick engine with terminal oil problems.
NEVER use 10W40 or 20W50. The additives used to provide such a wide viscosity range reduce the actual quantity of oil in the can, reducing lubrication qualities.
I use 10W30 year-round. No problems, and the temp where I live ranges from -10F to 100F.
If using synthetic:
Since lubricating qualities are better overall, you can (if necessary) use an oil with a wider viscosity range. Unless you are in extreme weather, though, 10W30 is always the best choice.
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