I encountered a weird downshifting issue on my s10 today. I went out for a long ride and about 120 miles in, the bike had trouble shifting from 2nd to 1st. No trouble going from 2nd to 3rd, 4th, etc. Only from 2nd to 1st. Riding in 2nd, I would pull the clutch lever all the way, press down on the shifter, and the lever would go down, but the transmission would not shift into 1st. I tried re-engaging the clutch, even giving it some gas, and then try downshifting again, but nothing.
Weird thing is that the problem was inconsistent. One stop light it wouldn't work. 2 minutes later at the next light everything shifts fine. 4 minutes later at the next light again no downshift.
For reference, I only have 1650 miles on my s10, of which I put on the last 700 miles. I didn't abuse the bike at all and mileage is really low for transmission failure.
From a quick search online I saw comments on oil and the shift pivot bolt. Modern oil is modern oil is modern oil, but who knows what the PO put in so I changed that out with a Bosch 3300 filter and Rotella T6. I would prefer an oem filter, but it's Sunday so I'm stuck with whatever is available from the local auto supply store. I haven't taken the s10 off-road yet so the shift pivot bolt was clean, but I wiped it off and relubed anyway.
Took the bike out for a 15 minute shakedown and the problem seems to be resolved and shifting is a bit smoother. If it comes up again, I'm taking it to the dealer and using the extended warranty.
Current: 2012 Yamaha Super Tenere, 2007 Honda CBR600RR
Past: 2006 Honda 919, 2004 Honda 919, 2007 Honda 919, 2008 Suzuki SV650S