When I 1st bought my bike, it came w/ the Honda GN4 10w-30 and honda filter.
about 1k miles later, TX summer heat was in full swing. And it was at that time that my motor started telling me very subtle things in which I wasn't hearing before. Shifting felt notchy, and I could hear it clunk loudly from gear to gear through my helmet. As a result, I was starting to think that I'm also hearing some gears whining while throttling up and down. I really couldn't tell for sure.
Then one day, I started it, and FOR SURE!- heard a new noise that sounded like cam chain noise. Like the cam chain is rubbing against the chain guide or against the side case. Well, long story short, I went with a thicker oil b/c it's just too darn hot down here to run 10w-30. And, one nice outcome is that the weird chain noise has never come back again.
So this is what I've been using for the past 1.5 yrs,
Yes, I could have bought the same viscosity in a m/c oil, but
I've had about 5 yrs of legacy data on this oil used in another m/c, and it's
held up unbelievably well. Far better than the M/C specific Bel-ray synthetic 10w-50 previously used in that m/c.
Also, this article I read:
http://www.ducatimeccanica.com/oil.html
reinforced my thoughts that there's more marketing than real data
to support a m/c specific oil. In addition, m/c oil has a HUGE markup. Dealers get them @ 40-50% below list price. So, when I pay $8-$10 a qt for m/c oil, am I getting 2x better protection than the car oil @ $4.50-$5/ qt?
I'm not advocating for anyone to go ahead and use what I'm using.
This works for me, so you got to decide what oil you're going to get w/ your money.
I'm just saying what's on my mind, really.