Synthetic oils cannot exacerbate your valve noise problem. As a matter of fact, some synthetic oils (Mobil 1, Valvoline SynPower, Castrol Syntec etc...) have very good cleaning additives which are designed to satisfy ACEA A3/B3 and higher criteria. These additives will help ease the stuck valve lifter problem (less noise
Be sure to use diesel-rated oils (CI4) for a diesel car and gasoline
rated oils (SL) for gasoline cars. You can use CI4 oils in gasoline cars too (all CI4 oils also pass SL spec).