Definitely fill up. If you're coming from the south, there's a petrol station on the outskirts of Oban, just before you hit the town proper.
Is there plenty of choice on the island? Will we have problems with Sunday closing if we arrive late on Saturday
No, there is not a lot of choice. The island's three (I think) Spars are a bit crap, and rather overpriced. If you're near Tobermory, the Co-op is usually open until late on a Sunday. There is no butcher in Tobermory any more - it closed down. However, there is the excellent Tobermory Fish Company, which is just out of town, and as well as fish they also sell very, very good meat (their venison is excellent). Also near Tobermory is the farm where they make and sell Isle of Mull Cheddar and other local foods, but last time I went, in early pandemic times, it was almost closed and a bit odd - maybe they have sorted out an M.O. now. There is a little supermarket in Dervaig that's quite good, but it's very expensive ... your friends are right, OP: do a big shop before you go.
What to do there? Get an OS map, and just walk. Get into the hills. Do that, and you won't see another living soul all day.
Go to Ulva, if they are running the little ferry. It's lovely.
Go to the very pretty white sand beaches at Croig.
A bit of a daft one, but ... the charity shop at Craignure is one of the best charity shops ever. Full of great books, and I got a new cashmere hoodie in there for £8!