Renewing O-rings when they must be removed and replaced, will make doubly sure the seal is maintained. The old one may seem O.K, but heat and pressure after tightening can make them brittle and hard.
Don't take any chances insist on new Oil Seals, O-Rings and Gaskets.
Pro-workshop gasket sealant: 08C70K0234M to complete the job like a professional.