I have replaced oil/filter few days ago but didn't have a chance to ride my ZX till today due to rain (I don't ride in rain anymore. Too many crazy drivers around Silicon Valley). I was using Castrol 06114 4T 10W-50 full synthetic before. Castrol was approximately $9 per bottle at Amazon. I used Valvoline 20W-50 full synthetic motorcycle oil and got those from Walmart. Each bottle was $5 or so. It could be because of the cold temperature this morning (40-42F), but the gear shifting was noticeably harder. After few miles, it got better but I still prefer Castrol over Valvoline. Both meet/exceed the same JASO2 and other certification. Maybe I should have used 10W-40 but I feel better with 20W-50 riding summer here, especially in heavy traffic. I guess I will go back to Castrol at the next oil change.

Because the lower number/viscosity is only in the low temp winter conditions both oils should have about the same viscosity based on the ambient conditions. Different bike oils really do seem to have a marked difference in the clunky shifting of the 14R. I found one that seems to work good, shifts good and after 3500 miles I don't notice any notchy feel or difference from last year when it was newer. I don't care about a few dollars difference for an oil change in the big picture of things. Ill stick with what works for me , Motul 300V 10W-40. Unless I decide to go into Death Valley in summer maybe .... nfw

Yes, my tranny is the clunkier than before for sure. I thought about getting Motul but it was bit of too much money. For the next oil change, I may try Motul as I see many good reviews about it.

Consider that 20W-50 is twice as viscous as 10W-40 when cold, and you can understand why the shifting is clunky.