8. As shown in Figure 3, in the Cartesian coordinate system $x O y$, 
the vertices of the polygon $O A B C D E$ are 
$O(0,0), A(0,6)$, 
$B(4,6), C(4,4)$, 
$D(6,4), E(6,0)$. If 
the line $l$ passes through the point $M(2,3)$, and divides the polygon $O A B C D E$ 
into two parts of equal area, then the function expression of the line $l$ is
8. $y=-\frac{1}{3} x+\frac{11}{3}$

As shown in Figure 9, extend $BC$ to intersect the $x$-axis at point $F$, connect $OB$ and $AF$, and connect $CE$ and $DF$ to intersect at point $N$.

From the given information, $M(2,3)$ is the midpoint of $OB$ and $AF$, meaning $M$ is the center of rectangle $ABFO$. Therefore, line $l$ divides rectangle $ABFO$ into two equal areas.

Since $N(5,2)$ is the center of rectangle $CDEF$, a line passing through $N(5,2)$ divides rectangle $CDEF$ into two equal areas.
Thus, line $MN$ is the required line $l$.
Let the function expression of line $l$ be $y=kx+b$. Then
$$
\left\{\begin{array}{l}
2k+b=3 \\
5k+b=2 .
\end{array}\right.
$$

Solving, we get $\left\{\begin{array}{l}k=-\frac{1}{3}, \\ b=\frac{11}{3} .\end{array}\right.$
Therefore, the function expression of the required line $l$ is
$$
y=-\frac{1}{3} x+\frac{11}{3} .
$$