Singer 99s are 3/4 size machines. About the same height as a full size machine but shorter and narrower. Chances are it is just old oil. You can download a manual at Singerco.com for free. They use class 66 bobbins and standard sewing machine needles.
Old oil can be cleaned quickly and easily with alcohol. Just do not get it on the finish. Alcohol will damage the shellac but is great for easily removed items like presser feet.
Congrats on your find.
Rodney