Thanks Nestor. Another site has this to say, which might be of interest to you. note that subject discussed there is an external door, more subject to seasonal movement. also note the possible negatives on urethane.
Thanks.
"In terms of work involved, the polyurethane will probably give longer life without period maintenance. However, if you let the finish deteriorate you will have to do a complete stripping of the door. Tung oil requires periodic maintenance coats, usually yearly, but these are relatively simple - just clean and wipe on more oil.
If you like a high gloss look, you will never get it with tung or linseed oil. You can get a lustrous sheen, but not gloss. Many woodworkers feel that a polyurethane gives a somewhat plasticy look.
If you go the "varnish" route, consider a spar varnish such as McCloskey's. A true spar varnish is made from natural resins. It forms a durable, flexible coating. If you have a panel door, the flexiblity is important. The panels in entry and garage doors move with the variations of humidity with the seasons. The panels are purposely not glued into their slots. If they were not given room to expand, they would crack. Polyurethanes are very hard and brittle. As the panels move, the bead of coating where the panel meets the rail or stile breaks, allowing water to work its way into the wood. Then the sun beats on the door and vapor pressure is created in the wood. Peeling results. Pay attention to the seam at the bottom of the panel where it meets the rail. This is where water damage will first become evident.
Over the years, I have finished many new fine garage doors of redwood, hemlock etc. Often the manufacturer would put a label on the door stating that if polyurethane was used on the door, the company would void its warranty
If you decide of tung oil, make sure you are using real tung oil. Many products labeled as "tung oil finish" actually don't even contain tung oil.
Many people do not like linseed oil as it can blacken the wood. Some manufacturers have stopped using linseed oil as the base for their stains for this reason. Linseed oil has nutriant value for mildew.
However you decide to treat your door, do not forget the top and bottom edges of the door, especially the bottom! The cross grain cut of the door rails will wick water up into the door leading to failure of the finish and warping of the door."