Why does Mercury recommend 10W30 engine oil on small outboards and 25W40 on larger outboards?
In fact, Mercury recommends 25W40 in all cases except the small outboards at low temperatures. At lower temperatures, oil tends to become thicker, resulting in more friction. On manual-start models, this additional friction can have an affect on the pull force required for starting the outboard. The 10W30 oil has less friction at cold temperatures and therefore results in a lower pull force for easier starting.
