The ornamental design for an oil funnel, substantially as shown and described.
FIG. 1 is a front perspective view of a version of the oil funnel showing the new design;
FIG. 2 is a front elevation view thereof;
FIG. 3 is a rear elevation view thereof;
FIG. 4 is a left side elevation view thereof;
FIG. 5 is a right side elevation view thereof;
FIG. 6 is a top plan view thereof;
FIG. 7 is a bottom plan view thereof; and,
FIG. 8 is cross-sectional view taken along 8-8 of FIG. 6.
The broken lines shown in the drawings represent environmental matter and form no part of the claimed design. The design is not limited to the scale shown herein.