8. (5 points) The unit digit of the sum of the product of 2016 2017s and the product of 2017 2016s is

保留源文本的换行和格式，翻译结果如下：

8. (5 points) The unit digit of the sum of the product of 2016 2017s and the product of 2017 2016s is
Following the cycle of 7, 9, 3, 1 every 4 numbers,
$$
2016 \div 4 = 504
$$

There is no remainder,
so the unit digit of the product of 2016 2017s is 1;
Similarly, the unit digit of the product of 2017 2016s changes: 6, 6, 6, 6,
the unit digit of the product is always 6;
Therefore, $1 + 6 = 7$;
Thus, the unit digit of the sum of the product of 2016 2017s and the product of 2017 2016s is: 7; hence the answer is: 7.