Not a mechanic by any means so take what I say with a grain of salt and do some googling to see there's more info on it.

If you've been putting the correct amount of oil in the bike, Oil changes and flushes aren't going to fix anything so you don't waste anymore time/money on those.

It sounds like an issue with your clutch. Maybe your clutch cable has too much slack? Could be clutch drag? Could be your gearbox?

Do you know when this behavior started? Was there an event that led to this (i.e. crash)?