Any object that will scratch a line into paint to reveal a colour beneath can be used.
All drawings of nature are inventions
You have to play around with oil pastels and see if they fit you.
Oil sticks are like oil painting, but free, wild and unpredictable. You always start from scratch. There is no need to succeed at the very first attempt at oil-pasteling your life. I like oil pastels because they're just ready to go. Just using both sgraffito drawings and oil pastels is not interdisciplinarity.