Quote:
Originally Posted by 1994Ninja250F
The “W” number isn’t important unless you’re riding in the wintertime.
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This is wrong. W for wrong.
The W might stand for winter, but it's really the viscosity grade of the base oil. Viscosity modifiers help the oil to "thicken" as it warms and behave with a similar viscosity to the second, higher number. So a 10W-30 is an oil with a viscosity that falls into the SAE 10 grade; however, viscosity modifiers have been added to this oil so that when it is at operating temperature, its viscosity falls within the SAE 30 grade.
"Cold" is a relative term. Remember your engine easily sees 200F temps in the hotter places, IE the piston ring and cylinder contact area. New engines that run more boost, higher exhaust temps, and hotter thermostats are seeing higher internal temps than traditional engines like our 250's which were designed in the early 80's for the most part. For ISO standard measurements, viscosity is compared at 40C and 100C, where 40C is the "cold" measurement and 100C is the "hot" measurement. I don't remember SAE comparison temps off the top of my head, but they're not far off. 180F is ringing a bell for the hot temp.
The reason I bring up temperatures is because the "cold" temp is well above a "winter" temperature, even though the W does stand for winter. That means the number before the W is important for every cold start the engine will ever see. Remember that the temperature inside the engine between parts at the time is is running is what's important, much moreso than the ambient conditions when the engine is run.
The garage hack method for picking a multi-grade oil is to go as low as you can on the first number without getting excessive valve train noise at start up, and go just as high as you need on the second number to maintain oil pressure at your harshest operating conditions (internally).
The easiest way is to just listen to the manufacturers recommendations, in this case 10W-40. Although the bike does seem to respond really well to synthetic 5W-40.