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.