I have a 1999 Suzuki SV650S with 15k miles on the clock. Sometimes the gear/shift pedal is really stiff to down or upshift. About 70% of the time it shifts nice and smooth. I haven't been able to identify any pattern to this.
I had a go at adjusting the cable at both ends but this made no difference. Oil was changed about a year ago, fully synthetic. I bought the bike couple of years ago and don't remember having such an issue when I bought it, but remember hoping the oil change would help the gear change issue so probably had the problem for around 15 months. Sometimes the stiffness doesn't come for a week or so, sometimes on / off several times a day. I use my bike most days.