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According to the 2002 996 owners manual 10W 40 is one of the approved weights. I would assume that 99-01 is the same.
Yeah, things change. If you look at the newest Porsche A40 approved oil list for all Porsche vehicles (except the Cayenne which uses the C30 approval list) there is not a 10w-40 on the list for any region of the world.
These are the weights for approved Porsche A40 oils:
0W-40
5W-40
5W-50 (and this only applies to Mobil 1 5W-50)