Spotted gum for sure, merbau is blar... no grain or aesthetic appeal whatsoever. You have to use a stain so it does not fade but make sure you don't buy it at Bunnings because they do not stock any product that is worth putting on any timber. The two decks are Organoil in red stain and the railing on them is livos Alis in light teak. I use the livos on my own timber work but it is expensive. All this timber has been in the full elements for well over 10 years.

Deck base

I've coated many decks and the best advise I've been given is "leave it natural". Most timbers go grey after a few months and don't look too bad. If you want a bi-annual maintenance job the other bit of good advise I've had directly from the manufacturer is put down four coats minimum. The third and fourth coats won't use so much oil because they don't soak in. A very light sand between coats keeping in the direction of the grain is a good idea to retain a finish without wood grain or dust spoiling the final appearance. Two coats, no matter which brand or type of oil will last about a year in our sun.

Have you considered using an aluminium deck? :) DecoDeck combines the warm, natural appearance of real timber with the superior strength and durability of aluminium. Available in 6.5 metre decking boards, DecoDeck uses a concealed clip and screw system for fast and easy DIY installation. The strong aluminium boards mean joists can be spaced up to 600 mm apart, and the deck joiner allows decking boards to be joined between joists, minimising wastage! https://www.decorativeimaging.com.au/building-products/decodeck/