I claim the ornamental design for a oil filter, as shown and described.
FIG. 1 is a perspective view of an oil filter, showing my new design;
FIG. 2 is a top plan view thereof;
FIG. 3 is a bottom plan view thereof; and,
FIG. 4 is a front elevational view thereof; the rear elevational view, left-side elevational view, and right-side elevational view all being mirror images thereof.