@D2Cat has the ticket. Linseed oil. Use it in all the teak trim pieces on the boat. Including the swim platform. Once a year and the platform sits in water. Still golden brown after 20 years.
I did the same with my trailer that has treated lumber decking. Worked great and despite my concerns it did not drip, become slippery or leave residue on clothes when I sit on it. It was absorbed into the wood and it's been over a year and still looks great. I'm thinking next year I'll re-coat just for protection. I used Hyd. oil just because it didn't have that dark black used diesel oil look. Also used it to treat the siding extension boards on my dump trailer. A day after applying got a frog chocker rain and there was no runoff residue down the side of the trailer.I did the waste oil and diesel (50/50) on my new 18’ car hauler earlier this summer. The method I read about was to apply and let it soak in, and then let it sit in the sun for a few days to dry. It worked well for me, didn’t even leave an oily residue if I touched or sat on the deck.
To each their own, but 50 years ago only about half the roads here were paved so DOT routinely used 10 wheel tank truck sprayers to spray the roads with used motor oil to control dust. If me dripping a pint of oil on the ground while mopping it on a trailer deck is going to contaminate the drinking water, everyone here is pretty much already screwed anyway. That said, yes I turn in waste oil to be recycled.I don't know, but why would you even take that chance? It will pollute someone's water.