I would never consider using a 10W-40 oil in this age of synthetic oils. A quality full synthetic 0W-30 or 5W-30 will give you excellent start-up protection and once at operating temperature, excellent flow for cooling and extended wear protection. Today's 30 viscosity synthetics have much better high temperature shear strength than a conventional 40 viscosity oil. The only reason a 10W-30 oil exists is becasue they, before synthetic oil chemistry, couldn't easily make a 5W-30 oil. The only reason you can still buy a 10W-40 oil is because there are still some really old engines around whose manufacturers at the time recommended it. There is no good technical reason to use it.
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