A pertinent question to the current situation on my Sportster. Having the luxury of a mixed and sizeable stable, some of my bikes don't get to go out so much. On the Sportster, last year I was having increasing difficulty clearing the clutch each time it had been standing a while. With no kicker I was attempting this by engaging a gear and then pushing back and forwards with the clutch held in before then firing up. But it got to the point where even that wouldn't do it.
I've recently pulled it down - primarily to check the alternator magnets in the back of the clutch drum. A well known weak point in the very early Evos. I'm digressing here, but mine had gone walkabout as it happens. Dammit - v. expensive, and I suppose could have led to a sudden seizure that could have killed me. Coulda, woulda, shoulda - but didn't so get over it.
Anyway, on pulling the plates they were really sticky and difficult to split. All cleaned up again now, but the suggestion is that I've used the wrong oil in there and this problem may repeat itself where that leaches out of the friction material. They are the originals (bike at a genuine 21k miles) but in good condition, but I may yet have to swap them out for new. I guess we'll see once we get into riding season and I take the bike off SORN.