I first used Cetol (natural), but it was not very durable at all. It may well be a lot better if you use a coat of epoxy first. Despite the warnings!
Whitworths carry a two-part epoxy sealer which has consistency of water, even after mixing, and penetrates quite well. Leave it to dry for 4 days, then apply whatever you wish. Although I just noticed they say to only use a clear finish in permanently shaded areas. So You might need a tinted product, such as Cetol, before a clear gloss.
I have only used it on teak hatch trims, which I decided to paint.
https://www.whitworths.com.au/norglass-norseal-wood-treatment-clear
I have seen very good results for Awlwood. It has several shades, but can be a bit too reddish for some. I think its best left for professional's to apply as there are some tricks to getting it right.
Before I discovered the NorSeal I went the Deks Olje route. You apply multiple coats of #1, wet on wet, until no more can be absorbed. Then wipe off with a cloth. Allow 3 days to dry, then apply multiple coats of #2 with a day between coats. I'm reasonably happy with results. After 6 months a light rub with a green scourer pad using #1 oil. Then a couple of coats of #2. Its all quite painless! I believe #2 is a polyurethane.
What I am testing on one section of rail for the 6mth top-up is a light, dry sand of the rail, then Cabots Exterior Polyurethane 'Marine Grade' (from Bunnings). It seems more durable then the Deks Olje #2. At least a year, perhaps longer. Cabots by itself is durable enough but the wood gets bleached underneath, hence my preference for using Deks Olje first.
For my decks, I first used Starbrite Tropical Teak Sealer (light). Its Ok, but not great. Then I used the Teak Wonder system. Teak cleaner, then brightener and then multiple coats of Sealer. It is easy to apply but does no last very long. Worse, it leaches out with rain, and stains the gelcoat beneath the scuppers.
So now I'm using Deks Olje, #1 oil only. I thinks that's what I'll be sticking with.