We usually use the word "rating" for movies, and "grades" for something more academic such as exams (A/B/C, etc), but they can be used quite flexibly 在这种情况（电影的评价之类的）我们通常会用“rating",特别是如果有分数的话（比如十分内你会给多少分），如果在说学校成绩或跟学习/考试有关的题目会说”grade" （这通常不是分数，是A/B/C/不及格/等）,但这两个字的意思都很相近

It depends on what the context is on the sentence as a whole, such as, "You got a 77 for the exam." This would be most referred to as a grade rather than a rating. OR "I saw on Yelp this place got a 4 out of 5." This would be mostly referred to as a rating rather than a grade. In your case in 'A movie gets a 8 out of 10', most people would referred it as a rating than a grade. Both grades and ratings can be professionalized and opinion based meaning, for example, "A restaurant has received an A code from the ", this is more professionalized because an inspector thoroughly checks the restaurant and GRADES them on how clean the restaurant is. "This restaurant has received an 4 out of 5 on Yelp", this could be either professionalized AND opinion based by a food critic or just a regular person who ate at the restaurant. You just really have to read the sentence and understand what it is just based off the context. Sorry this was long hope it helps.

They are very helpful, thank you!

Rating is most common for movies. Grade is okay, but usually grades are used for school work evaluations.

Both are okay, but I often hear and see rating more. A grade can also be A, B, C, etc (letter grades), which is sometimes used to review movies and music.