Did you get a manual? There are 3 manuals at
https://ismacs.net/singer_sewing_mac...e-manuals.html but 2 of them have different stitch length regulators. Yours is like the one in the 99-13. There are oiling instructions in the manual. Basically oil where metal moves against metal.
You want to clean the machine head with sewing machine oil as other products are likely to ruin the decals. Only use sewing machine oil, not 3-in-1 or WD-40. Also some good resources/videos can be found at
How to Clean Up and use a vintage sewing machine - videos by Muv and Fav
Have fun.