noise when reving the engine from iddle and oil type to use

[328ride]

Hi all,

I just bought a 2000 2.7L 986 with 78000 miles.
I noticed a metalic valve clicking sound (similar to the sound of a typical VW beetle) when reving the engine from iddle to maybe 2000rpm. The sound seems to stop when I rev it higher. Otherwise the engine sounds "normal" under load and acceleration

The dealer (not Porsche) just did the oil change and I don't recall hearing this sound when I first tried the car. Keep in mind that I noticed this sound when the windows were down (they were up during the first drive since it was rainning). Therefore, I'm not sure if it has to do with th oil grade they filled the car with (still need to confirm with the mechanic) or if the sound was always present

My first guess is that this sound as to do with valves or parts related to them. Maybe incorrect fuel grade..

Is this unsual or normal?

I'm about to drive the car back home (4000km) and will appreciate any helpfull info before taking any chances.

And,
last newbie question: what is the proper engine oil grade? Synthetic 0w40?
Any other acceptable grade for a temporary use

Depending on what's to be used, is it mandatory to redo the oil change if they didn't use synthetic?

thanks in advance for the precious info.