Use the recommended 5w-30 oil and sleep well at night. Newer versions of that engine can run 20-weights, but the older ones recommended a 30 weight.
Is it an automatic or manual transmission? If it's automatic, be very careful what fluid goes into it, as a regular old Dex/Merc fluid will destroy the automatic in short order.
Change the fluids on it, and it should be a great little car for you. It'll easily get about 32-35 mpg highway and about 22-24 mpg city. It's a bit buzzy at highway speeds, but will gladly cruise right along for a long time. Replacement parts for that generation of Elantra aren't horribly priced, either. Hyundai had mostly sorted out their quality issues by 2003, and were starting to work on making their interiors better. IMO the 2003-2006 Elantras were the most driver-friendly Elantras because they had decent power with pretty good handling, and a functional and comfortable interior, if somewhat plasticky and spartan.