The tailgates on 7 and 9 series wagons are aluminum. This does not mean that they are immune to corrosion. I have seen a few with ugly surface "rust" spreading under the paint, which flakes off to reveal a white, powdery, deeply pitted surface that is just about impossible to repair. However, (unlike 240s) on most of the cars I've seen, the tailgates are in excellent condition for the age of the vehicle. I live in Northern Virginia, which is a moderate rust area---not as bad as New England but I've seen plenty of nasty looking cars around here.

From what I've seen, the 7/9 series tailgate corrosion usually begins with a scratch in the paint that lets salt come in contact with bare aluminum, or around/underneath the latch/handle area, rather than from the inside out, and not from moisture trapped between the window rubber and the metal, which is not a problem on the 7/9, as they have adhesive mounted glass. However, I don't see how an oil treatment of the inside of the tailgate could hurt anything, as long as the drain holes are kept clear.

If you are in the far North, I have heard of cases where the (aluminum) rear bumper becomes so badly corroded around the mounting bolts that it actually falls off the car, though I've never seen it myself.