I got's the FE350...and I use Mobil 1 15w50...it works flawlessly. Shifts like butter...and my magnetic drain plug is always MUCH cleaner when compared to using Castrol Racing 4T 10w50 synthetic in the past. That said the one time I had to use Rotella T4 15w40 dino oil in a pinch it too resulted in a cleaner drain plug. That's the short answer.
Side note: Never buy Castrol Racing 4T 10w-50 Synthetic. A 5 quart jug of Mobil 1 15w50 is less than $30, that includes tax, at Wally World...just picked up two more jugs on my lunch break today to make sure I got plenty for the Summer.
Long answer is depends on what the manufacturer suggests. If they suggest a 10w-50 and you can only find 10w-40 and 20w-50...then the latter is better...even if it's dino oil. Reason being the first number is the cold cranking performance...which is important. However, it's the 2nd number that is MOST important. According to Husqvarna the engine bearings expect to find an oil of a given viscosity at a given temperature. That is exactly what the 2nd number represents. The 2nd number matches the expectations of the bearings...in this case a SAE 50 flow at 100C. We have to assume the manufacturer knows what the real oil requirements are of course.
Had you gone with a 10w40...the oil at operating temperature would have had wrong flow to meet the requirements. To remedy the use of a higher cold cranking winter-weight, 20w vs 10w, you'd want to use a cold-cranking-procedure as if perhaps it was colder outside than it actually is.
My cold-crank-procedure is to hit the starter button for 10 seconds with the kill switch turned on...this allows the engine to spin without starting. This coats all the parts in oil prior to allowing the bike to start. After the 10 seconds of lubricating/priming procedure is done I turn off the kill switch and press the start button to start the engine. Once the engine starts I let it idle at least 10 seconds to be double certain everything is lubricated before driving.
That all said I don't get heavy handed with the throttle till I've gotten the oil good and hot....~15 minutes of riding. I'm anal about my beloved bike's maintenance if you can't tell. Bike was too expensive not to take care of it...below is voa on the 15w50. If there is a God...surely Mobil 1 15w-50 flows thru her veins.
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