I was about to buy a bottle of 3-in-1 for random stuff around the house, but I realized I have motor oil in the garage already.
Is there any harm in using, say, 5W30, for door hinges or whatever?
I use regular oil as well for things like you are asking. I would get a small pump oil can to help dispensing the oil. On hinges, to keep the mess down, remove the pin, coat it in oil, let the excess run off and reinsert the pin. You could still run a little over the surface of the hinge barrel but obviously you will need to keep a wipe rag handy.
Engine oil does have a lot of additives to reduce wear in internal combustion engines. Some of these additives might not be approved for domestic use. Personally I would use 3-in-1 or other light machine oil.
Use lard or vegetable oil on door hinges, not critical. But other applications like guns have specific requirements . Real gun oil is very high ,narrow boiling range, 3:1 Oil is pretend gun oil.There is low temperature vacuum grease that costs $ 30 for 25 grams. It depends on the application . Motor oil is often good enough around the house.