My preferred thread sealant for diesel or #2 oil is Hercules Megaloc. It is diesel rated and works very, very well and also cleans up easily. It also never fully hardens which makes slight fitting adjustments in the future doable without losing the thread sealing properties. Hercules Megaloc is available at most any good plumbing supply house but there are many diesel/#2 rated thread sealants on the market. The key is to use them correctly and not be sloppy.