Mine are always noisy after every winter storage, the remedy is to just get it hot and give it a good thrashing, get some pressure and temperature in it.
I've owned two z32's now, same procedure every year.
You could always start it up now and then but that's not really recommended either as the temps never get hot enough when idling so you _can_ end up with condensation in your engine which is not good.
As for oil Nissan prescribes a 10w-40, most oils in that category are semi synthetic. If you wanna run full synt you could run a 5w-40.
No problems regardless, almost any oil sold today will exceed the quality requirements for our engine.
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