The first thing I'd probably do is drop the coolant that is in there, flush out the cooling system the best you can and refill with fresh coolant. Top the oil up and make a note of the level and then ride it for a while, keeping a watchful eye on what's going on.

As you say, there are only two places where oil and coolant can mix: the head gasket and the water pump. If it's running OK, then the head is unlikely. Head gasket rarely fail on XTs, water pump seals do however. If the water pump oil or mechanical seal has failed then you'd expect to see coolant, oil or both leaking out of the tell-tale drain hole on the underside of the pump body. Wipe that area clean and keep an eye on anything coming out, if at all.

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