Doesn't really need any for protection.
A length of brown wood along the sheer line can emphasise it and impress the neighbours.
Do you want it to match any varnished teak trimmings elsewhere?
Mine was all nice and grey but after repairing a scarf there was a patch of brown.
I decided to scrape and sand and then slap on some cheap teak oil partly to make it all look the same, partly as a bit of grooming to a boat that was new to me ( nice boat, friendly boat) and partly as a displacement activity when I couldn't decide which of many jobs to do next.
In my case the only other bits of wood visible were the washboards, the tiller and the hatch cover - all in mahogany.
As a chippy of delicate sensibilities, I think varnished teak doesn't look much cop anyway - coarse grained and rather featureless, a bland brown. Which is OK by me when used as a contrast on something like a rubbing strake which I won't be seeing close to and more importantly is one of only a few bits of visible wood that will need maintenance.
Everybody has their favourite stuff for slapping on:
the natural oil content of teak is supposed to reject varnish films- don't know if this still applys to later imports of teak from different areas.
the Sikkens wood stain stuff is considered good but the solids content obliterates the grain. No great loss on teak.
there's expensive stuff mixed by foreign legionaires gone native in the jungle - I shuddered at the price once.
boiled linseed is cheap but slow to dry. Teak oil has some additives plus dryers.
Tar - of course - or black bitumen. No rafting up to you then.
Ive used Deks Oljie (sp) 1 for a while now and found it quite effective - easy to apply - not weather critical - does not skin, so prep easy when re-coating - gives quite good finish with 3/4 coats, but only lasts about 6 months :-(
When my current boat was being restored by Trouts of Topsham, I asked Mike Trout what i should do with the new rubbing stake and toe rails that he had fitted and the answer was to do nothing other than the odd scrub with the deck brush. Four years on and the wood looks as good as new.