Best to replace the oil with genuine Nissan OEM gear box oil. There's no gaskets on the plugs so no worries there, also always make sure you can remove the fill plug before you remove the drain plug.
Adding molyslip also helps with gear changes if you find them a bit notchy, 1st to 2nd is especially common.
Heres some quick links to a couple of our traders on here, others are available though in the traders section.
http://cougarstore.co.uk/index.php?main_page=product_info&cPath=1_12&products_id=83
https://h-dev.co.uk/product/nissan-350z-gearbox-service-pack/?v=79cba1185463