I don't know about Motul oils, but for Amsoil here is the difference
Pour Point (ASTM D-97)
10W-40 = -51 degrees F
20W-50 = -38 degrees F
Flash Point (ASTM D-92)
10W-40 = 453 degrees F
20W-50 = 464 degrees F
As you can see there isn't all that big of a difference.
Tom
Those who would give up essential liberty to purchase a little temporary safety deserve neither liberty nor safety.
Benjamin Franklin, US author, diplomat, inventor, physicist, politician, & printer (1706 - 1790)
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