8. (15 points) (As shown in Figure 1) In a $6 \times 6$ grid, each row and each column contains the digits 2, 0, 1, 5 exactly once. The empty cells separate the digits into four-digit, three-digit, two-digit, or single-digit numbers. The numbers on the right and below indicate the sum of the digits in that row or column, and 0 cannot be the leading digit of a multi-digit number. (Figure 2 is an example with the digits 1, 2, 3, 0 each appearing once) Therefore, the sum of all the digits on the two diagonals of the large square is.
18
【Answer】Solution: Observing the third column of the fifth row, we can see that 521, 152, or 150 must be adjacent. Then, according to the first row, 2015 must be adjacent. From this, we can determine the first row, the fifth row, and the third column (as shown in the figure). Combining this with the problem statement and careful consideration, we can arrive at the conclusion shown in the figure.

Since $5+5+5+1+2=18$, the answer is 18.