I believe it's just age geomut. When I redid my interior, I useda golden teak oil (Starbrite, I believe)and hand rubbed it in with a towel..I rubbed pretty hard and gave it 6 applications. When the last application "cured", there was the nice factory sheen left.
Pete, I don't think they ever used minwax, but I might be able to check with folks who are still there from that era. Minwax would be a less time consuming way to do it though and probably provide better protection.
I do remember when having to repair factory teak, that whatever we did to repair (and we didn't use minwax) matched perfectly