I applied bonded seals to all three oil tank connections which reliably stop leaking (https://en.wikipedia.org/wiki/Bonded_seal). They are originally called ,,Dowty washers" (called ,,Usit-Ringe" in german) and are available in different sizes. The surfaces should be as plane as possible. In my case slightly sanding was sufficient. O-rings are not reliable because of the shear force when tightening the bolt.