I bought a 4L thing of oil a while back for the first oil change, used half of it on the oil change then, and a bit more on something else.
When I topped off the oil, I used the second half, almost completely (without oil filter change, it's recommended using 1.5L) until the looking glass measure was at 1/2 to 3/4 between the 2 lines. Sound got muffled, nothing more. I've also been scared to actually ride it anywhere since the noise started, even after the oil toppup.
My friend said a complete oil change was a good idea, but if it wasn't a fix, I was out 20-40$ for another thing of oil, filter, and more time without riding.
The bike isn't in the best condition and I wasn't willing to spend more money and time on this one when I'm in the middle of crunch time for classes.
The diagnosis the guy gave me was a bottom end engine problem, with the bearings spinning due to oil starvation. And the possibility of piston damage. Nasty.
Everyone who came by and actually showed serious interest in buying it, were ready to fix it themselves or had arrangements ready to replace it.