The set screw on mine (same sight) fixed the problem you mentioned. My groups would be a little left one time, a little right another; I re-examined my grip, my squeeze and on and on until I realized that the rear sight was moving a bit. A quarter turn of that allen screw and it fixed the problem.
Interesting thread which lead me to do a bit more research on the reasons to disable the magazine disconnect. It appears that it had been already disconnected from my used 41 since it always dry fired without a magazine. Also, the disconnector just falls out of the gun when the right side grip...
I've used kerosene, mainly because I have some on hand and it is safe. It smells though. WD-40 smells much better and does a good job dissolving old crud. Let it sit for a while.
FWIW, I've found that pushing a jag with a patch through the barrel to be pretty effective at removing lead. With solvent of course. Use two patches if it's not a tight enough fit.