2006 Nomad-Upshift 1st to 2nd

[rez]

I'm still in the process of familiarizing myself with this bike. I'm noticing on 1st to 2nd upshift...approx. 50% of the time the shift is somewhat 'difficult'...some 'clashing/grinding' before it fully engages into 2nd. The other 50% it shifts perfectly. All other upshifts and all down shifts are no problem. Clutch operation seems fine to me. I'm not sure if oil brand/type makes a difference?? The previous owner changed oil just prior to me purchasing...it currently has Castrol 10w40 4 stroke (motorcycle) oil (non-synthetic). Bike has 77k kms. Am i doing something wrong? Is this a normal/common sort of thing? Appreciate any comments/thoughts. Thanks.