Unfortunately, I know of no clear finish that is tougher and more durable than gloss polyurethane. A tung or linseed oil will offer virtually no protection, particularly the latter. The oil will not peel, as it is not a coating but a penetrating finish....but the wood will rapidly deteriorate.
The real trick here, and the most likely cause of the previous peeling of the 'spar urethane' is poor surface preparation or poor preparation between coats. In that harsh environment, your door must be sanded back to clear wood, and the wood must be scrubbed of all impurities, using something like TSP, and then rinsed and rerinsed. Then out of direct sunlite and between the temperatures of 60 and 80 degrees F, apply a Flecto or Verathane gloss exterior urethane. Let dry and sand using 220 grit, then recoat, dry and sand using 320 grit or 3-0 steel wool, then a final light coat. Make sure to seal the underside and top of the door in the same way. Might want to consider adding a storm door with low 'e' glass to keep the damaging ultraviolet rays off the finish.
Perhaps others here have another method they like to use. But this way has worked for me on several occasions, and should give you several seasons of a good looking door.