But I have a question - why does the level of grain change from shot to shot?
Scenes with optical effects have more grain, this includes scenes with optical wipes.
This is completely correct. To do optical effects in a pre-digital age, it required physically stacking pieces of film into an optical printer.
So for the asteroid sequence for example, some shots had 25 separately filmed elements that would be combined optically into a single shot.
They were using positive separations (equivalent to Red, Green and Blue channels) so this would mean 75 separate passes through a traditional optical printer, with the grain getting multiplied on each pass.
Now Edlund and his team had developed a new kind of optical printer (the Quad) which halved the amount of passes required, but it is still a lot of passes, and each pass adds another layer of grain. (The Quad itself is a fascinating beast, and I could talk about it for an week and bore anyone to tears…just ask any of my friends).
Also, a lot of Empire is dark, and with 1980 film speeds, less light = more grain, and the film was shot on a variety of stocks, each with its own grain issues, so the film does vary a lot shot to shot.
No you would digitally grade and match the shots to be less jarring for an audience, and even in 1980 it was possible to do a better job of that aspect than was done for Empire, but as with Star Wars, they were under extreme time and money constraints, so the final grades and composites left a little to be desired.