It started an oily substance leak but oil level reads full. It soaked a piece of cardboard and gravel underneath in one night.

It started an oily substance leak but oil level reads full. In one night it totally soaked a huge piece of cardboard and gravel underneath. Oil pan and other parts of car are wet as well but as I said oil level reads full. What other things can cause an oily substance leak?

My car has 140000 miles.
My car has an automatic transmission.

– It started an oily substance leak but oil level reads full. It soaked a piece of cardboard and gravel underneath in one night.