Try some Hoppes 9 or even gun oil with a cloth and see how much of it will wipe off, soaking it in either for a bit might help loosen it up also. For any spots that do not just wipe off try a toothbrush or 0000 steel wool with some gun oil.
By the way, leaving a gun in a leather holster is just about the worst thing you can do for the finish, especialy in hot humid weather inside a car. Even stainless steel will rust in these circumstances. As Jason said if there are any spots where the bluing is gone you can touch it up with cold blue.
This is advice from a sage.
"He who laughs last, laughs dead." Homer Simpson