Let me explain the strangeness. I was changing my brake and clutch fluid today (first time I've ever owned a bike with mineral clutch oil) and while doing my clutch when the fluid got a bit above the low mark, almost all clutch lever resistance went away! There was still fluid in the reservoir but in order to get more to pump out, I had to add fluid a well as flick the lever to build enough pressure to pump the old fluid out. After doing that enough to be satisfied that all the old fluid was out and topping off the reservoir, the clutch resistance seems to be normal. Is this typical with mineral oil? It seems like it should act just like brake fluid, in as much as long as there is enough fluid in the reservoir to have the same resistance throughout.

It has to do with atmospheric pressure. All fluid motion will react accordingly.