Yes, this can be caused by the oil level being low, or the wrong type of oil being in the bike, but generally these things (or a dragging clutch cable) will cause slipping all the time. Slipping from a worn out clutch or glazzed clutch plates usually starts under throttle in high gear, then get progressively worse until it does it all the time. So chances are it is time for a clutch. The average clutch usually lasts longer than 13K, but I've seen the original clutch go out a lot quicker than that depending on riding style, the type of terrain you live in, how much traffic you ride in, and several other factors. And if the clutch cable was, at some point, too tight, or someone tried to do a burn out, or a previous owner was too gentle letting the clutch out, that could glaze the clutch plates, which makes them too smooth even if they are not worn out yet.
A clutch is not hard to do yourself.
HERE is a write up and some photos of me putting a clutch in a Suzuki Intruder. Your bike is different, but not radically different, so it will give you and idea of what is involved.