Teak Oil to keep it looking good - and remember to keep the teak clean, and reapply oil during the season. Teak won't rot significantly if its' left untreted as its' full of natural oils - will just fade to light grey colour.
Just have a look at the thread "Blotchy teak" down the page, and read, and re-read the post by Ianwright.
"Never use anything harder than a sponge on teak!"
Teak is NOT a hard wood, and will be damaged by anything harder than ... right: a sponge!
Of course never use high (or low for that matter) pressure water cleaner either.
Just clean it as often as possible with sea water, lots of sea water, and a very soft, natural sponge.
My own teak deck has just been redone after 24 years, and that's just how I shall maintain it!
Our teak shower tray was treated with Deks Olje D1 about 8 years ago. It still looks absolutely perfect. Fair enough, it doesn't see the light of day very often but it has quite a hard life. DO soaks into new wood extremely well, just keep on and on and on and.....
A "hundred years or so ago", there were no teak decks, or very few of them, and then tens of inches thick, and not ten or twelve mm. as they are today.
And "a hundred years or so ago", even, the nearly universal pine decks were not brushed, rather sanded with brick ... and a lot of water!
"A hundred or so years ago", the purpose of the deck was to keep the water out of the "accomodations", today, the purpose of a teak deck is just aesthetic.
My own teak deck, originally 15 mm. thick on 15 mm. marine ply lost 10 mm. in 23 years at the most exposed spots, althought I never used to brush it, except during the first year, or two .... due to ignorance!
I always use natural sponge, or one of the spanish floor sweeping thingy, made of cotton bands.
It's amazing to see the color of the teack restoring itself at the beginning of the sailing season, just with the sea water running free on the deck.
Couldnt agree more about using only salt water and lots of it regularly and sorry to hear bout your loss of 10mm teak over the last 23 years though you have to admint that aint bad at all.