When I was getting my boat ready for the year last year I had to bleed the trim tabs because one of them would not hold pressure. I found oil coming out thru the top of the transom on the inside. One of the hydraulic transfer tubes (Aluminum) for the trim tabs ( K planes) corroded away and was leaking oil into the transom. Would this cause the transom to rot? What should I do about the transom? I replaced the tubes with a stainless one and the holes that did not have the coroded tubes were dry.

If oil got in there, then moisture can too. Where wood and moisture come in contact, the wood will rot.